What a pity! Considering his intelligence and diligence, he ________ a higher grade.[ ]

A. can have achieved  

B. must have achieved

C. need have achieved  

D. might have achieved

两台PC机通过其串行接口直接通信时，通常只需使用下 * * 根信号线，即( )。

A．TXD，CTS，GND

B．TXD，RXD，GND

C．DTR，RTS，GND

D．DSR，CTS，GND