This platform would collapse if all of us _ on it.
A.stand B. stood C. would stand D. had stood
选B
句意：如果我们都站上讲台,它会倒塌的.
与现在相反的假设（虚拟语气）.所以if从句谓语选过去式.
又如：
If he were here, he would help me.
如果他在这儿,他会帮助我的.