Answer:
did not understand the lessons he gave them
Explanation:
flustered- agitated or confused
It cannot be A because he would be happy that his students went through lessons quickly, not flustered.
It can't be B because his students would usually not be learning from him if they were more skilled than he was.
It can be:" did not understand the lessons he gave them " because any teacher could feel that way.
It won't be D because that is not even relevant to the point.
