Does Giving an Incentive Really Work on Improving Exam Scores

Multiple Choice

Does giving an incentive really work on improving exam scores? For example,consider the following treatments to measure the use of incentives. Treatment 1: No incentive
Treatment 2: Incentive given to all
Treatment 3: Incentive given only if a certain score is received
For this experiment,assume all subjects are seniors at a university.If a block design were used,what would be an appropriate blocking variable for this study?

A) Exam
B) Treatment
C) None of the above