— 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

用直接法配制标准溶液的物质，称为（）。

A、阳性物质

B、阴性物质

C、中性物质

D、基准物质