The class could not keep a straight face on hearing the strange pronounciation of the new teacher. A. remain serious B. remain unaffected C. remain silent D. remain indifferent

The correct answer is A. remain serious.

The other options are incorrect because:

• B. remain unaffected is incorrect because the class was clearly affected by the strange pronunciation of the new teacher.
• C. remain silent is incorrect because the class was not silent, they were laughing.
• D. remain indifferent is incorrect because the class was clearly not indifferent, they were amused by the strange pronunciation of the new teacher.

The class could not keep a straight face on hearing the strange pronounciation of the new teacher. This means that they were amused by the teacher’s pronunciation and could not help but laugh. This is an example of a situation where people are unable to remain serious because they are amused by something.