I called ________ his house, but he wasn't in. So I left a message telling him I would call ________ him the next day.

A.on; on

B.at; at

C.at; on

D.on; at

答案及解析： C

解析：

导解：call on sb. 意为“拜访某人”；

call at sp. 意为“拜访某地”；

call up/call sb. 意为“给某人打电话”．