题目

According to the report，the number of animals used in experiments ________ by 30% over the years. A．was reduced  B．has been reduced C．were reduced  D．have been reduced 答案：B　[时间状语over the years常与现在完成时连用；“the number of＋ 复数名词”作主语，谓语动词用单数，且reduce与the number of animals之间是动宾关系，故选B。]

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