从所给的四个选项中，选择最合适的一个填入问号处，使之呈现一定的规律性（）
A．
B．
C．
D．
参考答案：B
22. We know a good deal about the people who lived in China 4, 000 years ago, because they ______write and leave written records for those who lived after them.
A．could
B．should
C．must
D．Shall
WWAN并不直接使用物理介质，而是基于已有的（）通信设施实现无线接入。
