— What would you do if it ------ tomorrow?

B.rains
if 在该句的用法是,如果主句中想要表达将来时态的意思,那么后面的if从句用一般时.
再举个例子：
The Dow Jones will continue to fall if the bill fails to pass tomorrow.
如果明天法案不能通过投票,道琼斯指数会继续下跌.