问题
选择题
If the Watergate Incident__________Nixon would not have resigned from the presidency.
A．did not occur
B．had not occurred
C．was not occurring
D．be circling
答案
B
had not occurred表示对过去的虚拟，条件句中要用过去完成时，主句中要用should(would，could，might)+动词原形。