— Oh, this is really a small old town.

— Do you know when it ______? [ ]

A. built

B. was built

C. has built

D. has been built

半双工通信指通信双方可以（）；全双工通信指通信双方可以（）。

A.同时发送

B.同时接收

C.发送信息，但不能同时发送

D.同时发送和接收