警察派了一名卧底去调查那个犯罪团伙。

The police sent an ___undercover agent to investigate the criminal gang.

这位记者进行了一次秘密调查，揭露了这起腐败案件。

The journalist conducted an ___undercover investigation to expose the corruption case.

卧底警察假扮成服务生来收集证据。

The ___undercover officer posed as a waiter to gather evidence.