Jack was in low spirits those days and his friends did all they could him up.
A. cheer
B. cheering
C. cheered
D. to cheer
网友回答
A. cheer
B. cheering
C. cheered
(答案→)D. to cheer
解析：考查不定式做目的状语：句意：杰克这些日子情绪不好，他的朋友极尽所能让他高兴起来。And后面的句子中all是先行词，they could是定语从句，前面省略关系代词，后面省略动词do，不定式在这儿是目的状语，选D。