单选题 Our manager is ______ an important customer now and he will be back this afternoon.
A. calling on B. calling in C. calling up D. calling for

【正确答案】 A

【答案解析】call on意为“访问，拜访”；call in意为“召集，召来”；call up意为“召唤，提出”；call for意为“提倡，要求”。所填之词的宾语是an important customer，所以应选call on。