if从句省略if，操作词倒装，只有三个操作词可以：had, were, 和 should.

所以，你首先得看出这里是省略了的if的虚拟条件句。然后你就知道只有使用should才能省略if，构成条件句。

1.This kind of work is hard and dangerous. But_________ , you would become rich.

A. would you succeed      B. could you succeed

C. might you succeed      D. should you succeed

老师我想知道为什么不用could？

     【答】如果 if 条件句的谓语中有 had, were 或者 should 之一，可以省略 if 后把 had, were 或者 should 移到主语的前面；但是，如果 if 条件句的谓语中有 could, might 或者 would 之一，不能省略 if 后把  could, might 或者 would 移到主语的前面。所以，But should you succeed = But if you should succeed。只有  D. should you succeed 可以作为本题的正确答案。