答案：A

题目分析：本题中存在if虚拟语气省略现象，还原为if it should rain,从句对将来进行虚拟，主句用should + do。故选A

点评：与现在事实相反，if从句用did（be动词用were），

主句用would/should/could/might/+do.

与过去事实相反，if从句用have + done，

主句用would/should/could/might/+ have done.

与将来事实相反，if从句用should + do；were to do；did，

主句用would/should/could/might/+do.

如果虚拟条件句的谓语部分有were,had 和should时，可省略if，把were，had和should提句首，变成倒装句。