某车间安排甲、乙两人共加工400个零件，甲与乙一起加工了4小时后，又由甲单独加工了6小时才完成任务，已知甲比乙每小时少加工2个零件，求甲、乙两人每小时各加工多少零件？

Sometimes we should      roles so that we can understand each other’s difficulties, thus promoting our cooperation.

A．occupy

B．imitate

C．assess

D．switch