Saturday morning I usually do my homework and the afternoon I usually play tennis.A. In;

问题补充：

Saturday morning I usually do my homework and the afternoon I usually play tennis.

A. In; on

B. On; in

C. In; at

D. On; by

答案：

A. In; on

(答案→)B. On; in

C. In; at

D. On; by

解析：在具体的一天，星期几或在具体的一天，星期几的上下午晚上，用on,在上下午晚上，那一年，季节，月份用in。根据句子的结构，故选B。