Question

17. ―What a pity! Their experiment is a failure.

―They    better under more favourable conditions.

A. should have done    B.  could have done

C.  must have done      D.  may have done

Answer 1

17.   B考查含蓄的虚拟条件句。句意："真遗憾！他们的实验失败了。""在更加有利的条件下，他们本能做得更好。"这里under more favorable conditions是一个隐含的虚拟条件句， 相当于if they had been under more favorable conditions。 这是和过去事实相反的虚拟条件句，所以主句用could have done的形式。could have done表示过去原本能够做某事而实际上并没有做。所以选B。