—I called you at 4:50 yesterday afternoon but no one answered.—Sorry. Iwith my friends at

问题补充：

—I called you at 4:50 yesterday afternoon, but no one answered.

—Sorry. Iwith my friends at that time.

A. swam

B. swim

C. was swimming

D. will swim

答案：

A. swam

B. swim

(答案→)C. was swimming

D. will swim

解析：句意：昨天下文四点五十分我给你大电话了，但是没有人接。——抱歉，那时我正在和我的朋友游泳。结合语境可知下文描述的是过去某时正在进行的动作，故用过去进行时态。选C。