题干
By the time you ____ in, I ____ two letters.
A：had come, had writte
B：came, wrote
C：come, have writte
D：came, had writte
答案(点此获取答案解析）
D
A：had come, had writte
B：came, wrote
C：come, have writte
D：came, had writte
中，若
是直角，则
一定是锐角．”的证明
过程如下：
，而
，
矛盾，所以上述假设不成立，