— Look! Somebody _____the sofa.

— Well, it wasn't me. I didn't do it.

A．is cleaning

B．was cleaning

C．has cleaned

D．had cleaned

明朝后期，北方出现“边境休息，东起延、永、西抵嘉峪七镇，数千里军民乐业，不用     兵革，岁省费什七”。造成这种局面的原因是： [ ]

A．鞑靼和瓦刺的首领先后接受明朝封号

B．明朝修缮长城，防止蒙古骑兵的南下

C．俺答汗与明朝修好，恢复封贡互市

D．加强北方边防驻兵，使蒙古不得犯边