flap

考试前他总是紧张不安。

He always gets in a ___flap before exams.

请把信封的封舌折好再粘起来。

Please fold the ___flap of the envelope before sealing it.

窗帘在微风中轻轻拍打的声音让我昏昏欲睡。

The soft ___flap of the curtains in the breeze made me drowsy.