He is______an important customer now and he will be back tonight.A.calling onB.calling upC

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He is______an important customer now and he will be back tonight.

A.calling on

B.calling up

C.calling at

D.calling for

请帮忙给出正确答案和分析,谢谢!

热搜:关于氨基酸代谢错误的论述是_________。