--I hear that a lot of scientific equipment has been bought

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--I hear that a lot of scientific equipment has been bought in our school
--How wonderful!Do you know when_____?
A it has been bought B had it been bought
C it was bought D was it bought
原因?其他三个答案为什么不能选?

c,社么时候买的,buy是个一次性动词.
因为有个when,问的是什么时候买的,就是买这个动作什么时候发生的,所以用一般过去时态,而do you know when .后面要用陈述句的顺序.

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