In communication, a smile is usually _____ strong sign of a friendly and _____ open attitude. [ ]

A. the, /

B. a, an

C. a, /

D. the, an

Dtru的功能有哪些？（）

A.信号调制解调和放大

B.突发形成、交织、加密和语音编码

C.时隙交叉连接

D.分集接收