audit

他对经济学感兴趣，所以决定旁听这门课。

He decided to ___audit the economics class because of his interest in the subject.

他对音乐很感兴趣，所以决定旁听这门课。

He was interested in music, so he decided to ___audit the class.

他对音乐感兴趣，所以决定旁听这门课而不拿学分。

He's interested in music, so he decided to ___audit the course without earning credit.