修建青藏铁路时遇到的主要困难有
①高寒缺氧②冻土广布③生态环境脆弱④环境污染严重[ ]
A．①③
B．②④
C．①②③
D．①②③④
答案：C
设一台PC机，内存配置为4MB，则此PC机所选用的CPU至少应该是 ( ) A) 8086 B) 80286 C) 80386 D) 80486
So far nothing ________ Tom.I want to know the reason ________ he hasn't written to me. [ ]
A．heard from；why
B．has been heard from；why
C．was heard from；that
D．has heard from；that
