题目

Experts have been warning ____ of the health risks caused by passive smoking. A. at a time    B. at one time  C. for some time   D. for the time   答案： C 句意: 专家们提醒注意被动吸烟给健康带来的危险已有一段时间了。for the time“暂时, 眼下”, 不合题意, 故排除D; at a time=on each occasion“每次, 逐一, 依次”, 不合题意, 排除A; at one time=at a period of time in the past“一度, 曾经”, 不合题意, 排除B; for some time=for a fairly long period of time“一段时间”, 符合题意, 故选C。

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