A good idea occurred to Mr. Baker to his invention recently.
A. While applying
B. While applied
C. While he was applying himself
D. While applying himself
网友回答
A. While applying
B. While applied
(答案→)C. While he was applying himself
D. While applying himself
解析：考查主语从句。句意：最近当布克先生专心于他的发明的时候。他突然想起了一个好主意。短语：apply oneself to doing sth 专心于某事，后面是一个时间状语从句，所以选C。