We'll go to visit the Great Wall ______it doesn't rain tomorrow.

We'll go to visit the Great Wall ______it doesn't rain tomorrow.
A、since
B、before
C、when
D、if
【正确答案】：D
【题目解析】：since 自从；before 在……之前；when 当；if 如果；句意表达的是如果明天不下雨，我们就会去长城。因此D 符合题意。