the days he had spent with his American friends he couldn’t help.A. Keeping up; smileB.

问题补充：

the days he had spent with his American friends, he couldn’t help.

A. Keeping up; smile

B. Staying up; smiling

C. Looking back on; smiling

D. Looking at; to smile

答案：

A. Keeping up; smile

B. Staying up; smiling

(答案→)C. Looking back on; smiling

D. Looking at; to smile

解析：考查非谓语动词，第一空考查现在分词作状语，用Looking back on回顾，第二空考查现在分词作宾语，can’t help doing情不自禁作某事，句意：回顾他与美国朋友在一起的那些日子，他情不自禁笑了。所以选C。