The given idiom is "keep a straight face", and it's used in situations where someone tries not to laugh or show amusement, even if something funny or amusing happens. In this context, the phrase was used to describe the reaction of the class upon hearing the new teacher's strange pronunciation. Let's analyze the options:
Therefore, the option that best expresses the meaning of the idiom "keep a straight face" in this context is Remain serious.
If the price of a commodity increases by 25%, by what percentage should the consumption be reduced to keep the expenditure the same?
A shopkeeper marks his goods 40% above cost price and offers a 10% discount. What is his percentage profit?