All the students gathered in the early morning _____ to pick up therubbish.

空内填 volunteering，用作状语，表示伴随情况。句意为：所有的学生一大早就集合在一起，自愿去捡垃圾。类似的用法如：

Don’t you sit there doing nothing. 别什么也不干坐在那里。

Everybody came in the room, singing and smiling. 大家唱着笑着走进教室。

He walked along in front of me, holding the lantern. 他提着灯笼走在我前面。

He stood helpless, not knowing what to do. 他无可奈何地站在那里，不知如何是好。

He combed through the files searching for evidence of fraud. 他详查档案，寻找欺诈行为的证据。

I offered them something to eat, thinking they might be hungry. 我给了他们一些东西吃，心想他们可能会饿。