问题补充：
The children went home from the grammar school,their lessonsfor the day.
A. finishing
B. finished
C. had finished
D. were finished
答案：
A. finishing
(答案→)B. finished
C. had finished
D. were finished
解析：their lessons finished for the day构成独立主格结构,独立主格结构是由一个相当于主语的名词或代词加上非谓语动词、形容词(副)词、或介词短语构成的一种独立成分。根据lesson与finish的关系可判断出要用动词+ed形式,表示被动。had finished和were finished都不是非谓语动词,所以不能构成独立主格结构。finishing是动词+ing形式,表示主动。
