—When did the Whites reach Beijing? —They reached Beijingthe afternoon of June 15. A. in B

问题补充：

—When did the Whites reach Beijing?

—They reached Beijingthe afternoon of June 15.

A. in B. on C. at

答案：

解析：在时间前，只要有天的用on，本题中有个15号，故本题选B。