龙胆草具有的功效是( )
A．夏枯草B．石膏C．淡竹叶D．栀子E．黄芩
参考答案：E
He hasn't heard from his friend ______ last month. [ ]
A. since
B. by the end of
C. for
D. until
简述I/O接口电路的电路单元。
