C 当主语一致,且有连词,可省略主语与be动词.主语he与reading为主动关系,所以填reading为什么read要加 ing因为省略了be动词，原句相当于while he was reading the book,he noddded from time to time为什么是reading而不是别的谢谢如果没有he，需用ing形式，AD不对，如果选B，应该是while he was reading the book,he noddded from time to time。while引导从句一般用进行时。