题目

 _______ (W) word came that three Chinese astronauts had been successfully sent into ______ space.      A. The; a                   B. 不填；不填               C. A; 不填                    D. 不填；the 答案：B18．已知m=$\sqrt{a}-\sqrt{a-2}$，n=$\sqrt{a-1}-\sqrt{a-3}$，其中a≥3，则m，n的大小关系为（　　）A．m＞nB．m=nC．m＜nD．大小不确定