— I came here by taxi and the driver _____ me 50 yuan.

— Really? You _____ by bus.[ ]

A. paid; should come

B. cost; should have come

C. charged; ought to have come

D. spent; ought to come

在计算机系统指令系统中，真正必须的指令数较少，其他指令是为提高计算机速度、提高编程灵活性而设计的。