问题描述：
有关倒装的选择 ------,our experiment wounld have failded
a if they didn't help us
b.if they haven't helped us
c.had they noy helped us
d.should they not helped us
我想知道b d 哪里错了?
a if they didn't help us
b.if they haven't helped us
c.had they noy helped us
d.should they not helped us
我想知道b d 哪里错了?
问题解答：
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