请帮我解出这道选择题，求解析，O(∩_∩)O谢谢。

Question

19. _____ it rain tomorrow, we would have to put off the visit to the Yangpu Bridge.
　　A. Were B. Should C. Would D. Will

Answer 1

在虚拟语气中，有省略if的用法，但此时应用倒装句型，即把从句中的were,should,had 等提到句首：
这句话的原句为If it should rain tomorrow, we would have to put off the visit to the Yangpu Bridge.
将if省略，should前置，就成了Should it rain tomorrow, we would have to put off the visit to the Yangpu Bridge.
　　
又如：
1. Were I Tom,I would refuse. 如果我是汤姆，我会拒绝。
2. Should it be necessary,I would go. 假若有必要，我会去的。
3. Had it not been for the bad weather we would have arrived on time. 若不是天气坏，我们就准时到达了。

Answer 2

你好，选B
原句应是If it should rain tomorrow,we would have to put off the visit to the Yangpu Bridge.
倒装后为Should it rain tomorrow,we would have to put off the visit to the Yangpu Bridge.
所以B
虚拟语气倒装 仅should were had 提前，其他不动。were to 与should等效用于对将来的虚拟。were，had分别表对现在、过去的虚拟。具体见虚拟语气用法及虚拟语气倒装。

Answer 3

选B.考查虚拟语气了。与将来事实相反，从句用 should + 动词原形，主句 would / should / might / could + 动词原形
另外，当虚拟条件句的谓语动词含有 were, should, had 时，if 可省略，而将 were, should, had等词置于句首。_____ it rain tomorrow，全句应为If it should rain tomorrow