乘客们一直在抱怨航班延误。

The passengers kept ___complaining about the flight delay.

居民们向市政府提交了一份投诉信，反映噪音问题。

The residents submitted a ___complaint letter to the city council about the noise issue.

她不停地抱怨膝盖疼。

She kept ___complaining about knee pain.