Idioms and Phrases - Verbal and Reasoning

In the following questions four alternatives are given for the idiom/phraseitalicised and underlinedin the sentence. Choose the alternative which best expresses the meaning of idiom/phrase.
Q1:

The class could not keep a straight face on hearing the strange pronunciation of the new teacher.

A remain silent

B remain serious

C remain mute

D remain disturbed

ANS:B - remain serious

No answer description is available.