--When will Mr. Black come to Beijing? the morning of September 5th.A. InB. OnC. AtD. F

问题补充：

--When will Mr. Black come to Beijing?

-- the morning of September 5th.

A. In

B. On

C. At

D. For

答案：

A. In

(答案→)B. On

C. At

D. For

解析：句意：布莱克先生什么时候来北京？九月五号的上午。在具体某一天的上午，下午或晚上用介词on。所以选B。