A
If the whole class earned A then they earned A and the average woud be A.
"C" is average. How could the whole class be above average. The teacher set the bar too low.
"A" = Way above average
"B" = Good, better than average
"C" = Average "
D" = Below average
"F" = Failure
So you are saying the average grade was above average ...
Calling the average grade an "A" is grade inflation defined.