Performance measurement

Sir please help me with the following:

Q. 1 A government is looking at assessing state school by reference of both financial and npn financial factors, one of which is average class sizes. Which of the three E’s best describe the above measure?
A. Efficiency
B. Effectiveness
C. Economy.

Sir answer is efficiency.. But what is is meant by average class sizes? Is it no of students in each class? And how its is eeficieny.. Efficiency is a measure which looks at input output relationship. How avg class size look at input output relationship?