I wrote a letter to John for his help during my high school years，without which I the college entrance exam.
A. would fail
B. would have failed
C. should fail
D. must have failed
网友回答
A. would fail
(答案→)B. would have failed
C. should fail
D. must have failed
解析：B考察虚拟语气，句义：我给John写了一封信，感谢他在高中给我的帮助，没有他的帮助，我不会通过高考的。句义可知，这是对过去情况的假设，所以主句使用情态动词+have done的形式。故B正确。