—What do you think we can do for our aged parents?

—You __________ do anything except to be with them and be yourself

A．don’t have to

B．oughtn’t to

C．mustn’t

D．can’t

列车管定压为500kpa时，列车管减压140kpa，制动缸压力应为（）。

A.240—260kpa

B.280—340kpa

C.340—360kpa

D.420—450kpa