A lot of experiments have been conducted________this product is put out on the market.

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问题 A lot of experiments have been conducted________this product is put out on the market.

选项 A、before
B、when
C、as
D、after

答案A

解析 主句提到已经进行了多次实验,从句提到该产品投放到市场。由句意可知,主句和从句之间存在时间上的先后关系。A项before意为“在……之前”,可以引导时间状语从句且符合句意要求。when意为“当……时”,as意为“作为”,after意为 “在……之后”。
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