题目

--How long ___________ each other before they__________ married? --For about a year. A. have they known;getB. did they know; were going to get C. do they know;are going to getD. had they known; got 答案：D 解析: 根据句意分析，他们结婚是已过去的事，那么，他们互相了解大约一年的时间肯定发生在结婚前，发生在过去某一动作之前的动作，应该使用过去完成时。下列各式恒等变形属于因式分解的是 [     ]A.（x-3）（x+3）=B.C.D.