snicker|snickered|snickering

听到这个荒谬的借口，她忍不住发出一声窃笑。

She couldn't help but let out a ___snicker at the ridiculous excuse.

听到老师念错了单词，几个学生在下面偷偷地笑。

A few students ___snickered when the teacher mispronounced the word.

当我打喷嚏时，我听到教室里有人在窃笑。

When I sneezed, I heard someone ___snickering in the classroom.