Helen _______her keys in the office so she had to wait until her husband ___home.

A．has left; comes

B．left, had come

C．had left; came

D．had left; would come

设有4个作业同时到达，每个作业的执行时间均为2h，它们在一台处理器上按单道式运行，则平均周转时间为( )。

A．1h

B．5h

C．2.5h

D．8h