题目

When I was in primary school, I got into a major argument with a boy in my class．I can’t  16 what it was about, but I have never forgotten the   17   I learned that day． I was  18   that I was right and he was wrong--and he was sure that I was wrong and he was right．The   19   decided to teach us a very important lesson．She   20   both of us up to the   21   of the class and   22   him on one side of her desk and me on   23   ．In the middle of her desk was a large, round object．I could   24   see that it was black．She asked the boy wha已知a，b，c，d是四条不重合的直线，其中c为a在平面α上的射影，d为b在平面α上的射影，则（　　）A、a∥d?a∥bB、a⊥b?c⊥dC、a∥b?c∥dD、c⊥d?a⊥b