The answer can't be A because to get the average of the class you need two things.
1. Average scores
2. Number

This is a weighted average problem.

If you add averages the answer is 161. Average of the two can not be higher than either the two numbers. Has to be in the middle, depending on which class has more people or weight in other words

1 is insufficient

2 by itself doesnt tell us B's average

Together you sill don't know the number of people


Try a few examples and you will understand

if A has 1 person who scored 79.5 and B had 26 with average of 80.5, weithed average = (Average A * #A + Average B*#B)/Total #

or A is 2 people and B has 27. Using the formula above you will get a different result.

Hope this helps

Thanks Sher1!

Wish you all the best!