复合式衬砌的隧道地质和支护状况观察量测的间隔时间是（）。
A.3次/d
B.2次/d
C.1～2次/d
D.每次爆破后进行
参考答案：D
— Did you find the missing couple in the mountain yesterday?
— No, but we ____ to get in touch with them ever since. [ ]
A. have tried
B. have been trying
C. had tried
D. had been trying
级数的收敛性是（）。
A.绝对收敛
B.条件收敛
C.等比级数收敛
D.发散
的收敛性是（）。