如图所示，静止在光滑水平面上的斜面体，质量为M，倾角为α，其斜面上有一静止的滑块，质量为m，重力加速度为g．现给斜面体施加水平向右的力使斜面体加速运动，若要使滑块做自由落体运动，图中水平向右的力F的最小值为（ ）

A. B. C. D. Mg

California has lost half its big trees since the 1930s, according to a study to be published Tuesday and climate change seems to be a major factor(因素).
The number of trees larger than two feet across has declined by 50 percent on more than 46, 000 square miles of California forests, the new study finds. No area was spared or unaffected, from the foggy northern coast to the Sierra Nevada Mountains to the San Gabriels above Los Angeles. In the Sierra high country, the number of big trees has fallen by more than 55 percent; in parts of southern California the decline was nearly 75 percent.
Many factors contributed to the decline, said Patrick Mclntyre, an ecologist who was the lead author of the study. Woodcutters targeted big trees. Housing development pushed into the woods. Aggressive wildfire control has left California forests crowded with small trees that compete with big trees for resources(资源).
But in comparing a study of California forests done in the 1920s and 1930s with another one between 2001 and 2010, Mclntyre and his colleagues documented a widespread death of big trees that was evident even in wildlands protected from woodcutting or development.
The loss of big trees was greatest in areas where trees had suffered the greatest water shortage. The researchers figured out water stress with a computer model that calculated how much water trees were getting in comparison with how much they needed, taking into account such things as rainfall, air temperature, dampness of soil, and the timing of snowmelt(融雪).
Since the 1930s, Mclntyre said, the biggest factors driving up water stress in the state have been rising temperatures, which cause trees to lose more water to the air, and earlier snowmelt, which reduces the water supply available to trees during the dry season.
【1】What is the second paragraph mainly about?
A. The seriousness of big-tree loss in California.
B. The increasing variety of California big trees.
C. The distribution of big trees in California forests.
D. The influence of farming on big trees in California.
【2】Which of the following is well-intentioned but may be bad for big trees?
A. Ecological studies of forests.
B. Banning woodcutting.
C. Limiting housing development.
D. Fire control measures.
【3】What is a major cause of the water shortage according to Mclntyre?
A. Inadequate snowmelt. B. A longer dry season.
C. A warmer climate. D. Dampness of the air.
【4】What can be a suitable title for the text?
A. California's Forests: Where Have All the Big Trees Gone?
B. Cutting of Big Trees to Be Prohibited in California Soon
C. Why Are the Big Trees Important to California Forests?
D. Patrick Mclntyre: Grow More Big Trees in California

1921至1922年苏俄农民要上缴的实物税有18种。1921年苏俄约有79个省级行政单位，完税后能实现自由交换和买卖粮食、饲料的省份，约占总数的一半多，而能够自由交换和买卖马铃薯、干草的省份各只有16个。这反映出，新经济政策实施初期（   ）

A. 余粮收集效果良好                                                B. 数省保持经济独立地位

C. 农民税收负担较重                                                 D. 农民无权支配税后粮食

利用下列装置和试剂进行实验，不能达到实验目的的是
A．A
B．B
C．C
D．D

Marian Bechtel sits at West Palm Beach’s Bar Louie counter by herself, quietly reading her e-book as she waits for her salad. What is she reading? None of your business! Lunch is Bechtel’s “me” time. And like more Americans, she’s not alone.

A new report found 46 percent of meals are eaten alone in America. More than half（53 percent）have breakfast alone and nearly half（46 percent）have lunch by themselves. Only at dinnertime are we eating together anymore，74 percent，according to statistics from the report.

“I prefer to go out and be out. Alone，but together，you know？”Bechtel said，looking up from her book. Bechtel，who works in downtown West Palm Beach，has lunch with coworkers sometimes，but like many of us，too often works through lunch at her desk. A lunchtime escape allows her to keep a boss from tapping her on the shoulder. She returns to work feeling energized. “Today，I just wanted some time to myself，”she said.

just two seats over，Andrew Mazoleny，a local videographer，is finishing his lunch at the bar. He likes that he can sit and check his phone in peace or chat up the barkeeper with whom he's on a first-name basis if he wants to have a little interaction（交流）. “I reflect on how my day's gone and think about the rest of the week,” he said. “It's a chance for self-reflection, You return to work recharged and with a plan.”

That freedom to choose is one reason more people like to eat alone. There was a time when people may have felt awkward about asking for a table for one，but those days are over. Now，we have our smartphones to keep us company at the table. “It doesn't feel as alone as it may have before al the advances in technology,” said Laurie Demerit， whose company provided the statistics for the report.

28. What are the statistics in paragraph 2 about?

A. Food variety.

B. Eating habits.

C. Table manners.

D. Restaurant service.

29. Why does Bechtel prefer to go out for lunch?

A. To meet with her coworkers.

B. To catch up with her work.

C. To have some time on her own.

D. To collect data for her report.

30. What do we know about Mazoleny?

A. He makes videos for the bar.

B. He’s fond of the food at the bar.

C. He interviews customers at the bar.

D. He’s familiar with the barkeeper.

31. What is the text mainly about?

A. The trend of having meals alone.

B. The importance of self-reflection.

C. The stress from working overtime.

D. The advantage of wireless technology.

Michel Crozier, with ________ help Daisy passed the exam, was one of the straight A students of the school.

如图所示，图中五点均在匀强电场中，它们刚好是一个圆的四个等分点和圆心。已知电场线与圆所在平面平行。下列有关圆心O和等分点a的电势、电场强度的描述中正确的是（ ）

A. a点的电势为6V
B. a点的电势为-2V
C. O点的场强方向指向a点
D. O点的场强方向指向电势为2V的点

甲醇是重要的化工原料，在有机合成中具有广泛应用。
I．(1)用甲醇制取甲胺的反应为 △H
已知该反应中相关化学键的键能数据如下：

共价键	C―O	H―O	N―H	C―N
键能／kJ·mol-1	351	463	393	293

则该反应的△H=__________kJ·mol-1
Ⅱ．一定条件下，将2mol CO和6mol H2通入2L密闭容器中发生如下反应
主反应： △H<0 Ⅰ
副反应： △H<0 Ⅱ
反应到t min时，达到平衡状态。平衡时CH3OH的体积分数φ(CH3OH)如图所示：

(2)图中a___b(填“大于”或“小于”)。图中Y轴表示温度，其理由是__________
(3)若反应II的平衡常数K值变小．则下列说法中正确的是___________(填序号)。
A．平衡均向正反应方向移动
B．平衡移动的原因是升高了温度
C．达到新平衡后，φ(CH3OH)减小
D．容器中φ(CH3OCH3)增大
(4)平衡时，M点CH3OH的体积分数为12.5％，c(CH3OCH3)=0.1mol·L-1，则此时CO的转化率为________；用H2表示I的反应速率为______mol·L-1·min-1。反应Ⅱ的平衡常数K=___________(用分数表示)

如图所示，一简谐横波在某区域沿x轴传播，实线a为t=0时刻的波形图线，虚线b为t=△t时刻的波形图线。已知该简谐横波波源振动的频率为f=2.5Hz，虚线b与x轴交点P的坐标为xP=1m。则下列说法正确的是( )

A. 这列波的传播速度大小一定为20m/s
B. 这列波一定沿x轴正向传播
C. 可能有△t=1.25s
D. 可能有△t=1.45s
E. 若该列波遇到宽度为6m的障碍物能发生明显的衍射现象

腈水合酶（ N ₀ ）广泛应用于环境保护和医药原料生产等领域，但不耐高温。利用蛋白质工程技术在 N ₀ 的 α 和 β 亚基之间加入一段连接肽，可获得热稳定的融合型腈水合酶（ N ₁ ）。下列有关叙述错误的是（　　）

A ． N ₁ 与 N ₀ 氨基酸序列的差异是影响其热稳定性的原因之一

B ．加入连接肽需要通过改造基因实现

C ．获得 N ₁ 的过程需要进行转录和翻译

D ．检测 N ₁ 的活性时先将 N ₁ 与底物充分混合，再置于高温环境

补写出下列句子的空缺部分。
（1）“达则兼济天下，穷则独善其身”是许多封建士大夫的人生信条，而范仲淹却在《岳阳楼记》中提出“____________，____________”，认为即使不得志也应心忧君国之事。
（2）《锦瑟》中以锦瑟起兴，引起对年华往事的追忆的句子是“____________，____________”。
（3）在《逍遥游》中表明大鹏如果没有大风的强大力量便无法展翅高飞的句子是“__________，__________”。

垂体位于脑的下部，因此也称为脑下垂体。成年人的垂体重约6g，由腺垂体和神经垂体两部分组成。回答下列问题
(1)腺垂体能合成和分泌多种促激素，比如____________等(答出2种即可)，这些促激素是它们所作用的____________________的形态发育和维持正常功能所必需的。
(2)神经垂体能释放________________和催产素，其中前者具有促进肾小管和集合管对水的重吸收的作用。这两种激素都是在下丘脑的神经细胞中合成的，却在垂体处释放到内环境，请写出合理的解释:____________________。
(3)垂体既能参与体液调节，也能参与神经调节。若某种物质也具有“既能参与体液调节，也能参与神经调节”的特点，则该物质可能是__________________。

他们不仅要把人从神的桎梏下解放出来，而且要把人从自然中解放出来。他们的政治活动，正是自觉或不自觉地执行着“人的社会解放是人自然解放的必要条件”。这里的“他们”推动了
A. 智者运动
B. 文艺复兴
C. 宗教改革
D. 启蒙运动

2018年2月13日，平昌冬奥会女子单板滑雪U形池项目中，我国选手刘佳宇荣获亚军。如图所示为U形池模型，其中a、c为U形池两侧边缘，在同一水平面，b为U形池最低点。刘佳宇从a点上方h高的O点自由下落由左侧进入池中，从右侧飞出后上升至最高位置d点相对c点高度为。不计空气阻力，下列判断正确的是

A. 从O到d的过程中机械能减少
B. 从a到d的过程中机械能守恒
C. 从d返回到c的过程中机械能减少
D. 从d返回到b的过程中，重力势能全部转化为动能

为虚数单位，复数，则（）
A. B. C. D.

(2018·全国卷Ⅱ)The Chinese Ministry of Agriculture finds that between 2005—when the government started a soil­testing program ________ gives specific fertilizer recommendations to farmers—and 2011, fertilizer use dropped by 7.7 million tons.

古往今来，社会关怀是一个重大议题。
16世纪上半期，德国许多地区和城市颁布法令，进行济贫改革。萨克森的莱斯尼希市设立济贫公共基金，将关闭的修道院的资产以及信徒捐给教会和修会的财物转入公共基金。公共基金的日常管理由10位民主选举产生的世俗人士负责。他们还接管了城市的济贫事务，统筹管理，对接受救济的对象进行资格审查，避免了教会和修会救济的弊端。
概述16世纪上半期德国济贫改革的内容及主要特点。

下列说法正确的是
A. 属于同分异构体
B. CH3COOCH2CH3与CHI3CH2COOCH3中均含有甲基、乙基和酯基，为同一种物质
C. C60与14C互为同素异形体
D. 乙酸与硬脂酸互为同系物

下列有关细胞生命历程的说法，正确的是
A. 与酵母菌相比，乳酸菌与外界进行物质交换的效率较低
B. 细胞衰老后色素减少、部分酶的活性降低
C. 被病原体感染的细胞的清除与细胞坏死有关
D. 精原细胞可进行有丝分裂和减数分裂

扇三角洲是由邻近高地推进到稳定水体中的冲积扇，下图为某扇三角洲示意图。据此完成下面小题。

【1】如图所示扇三角洲中，甲地与乙地沉积物的粒径大小规律是
A. 两地沉积物的粒径均值相似 B. 乙地无大孩粒沉积物
C. 乙地比甲地沉积物粒径均值大 D. 甲地比乙地沉积物粒径均值大
【2】如果图示扇三角洲前缘每年向水体方向推进0.5米，则说明
A. 图示河流年输沙量稳定 B. 图示河流含沙量稳定
C. 图示水体水位上升 D. 图示河流年输沙量增加