Answer:
Sarah wants to do well because of extrinsic motivation.
Joe wants to do well because of intrinsic motivation.
Step-by-step explanation:
Credit is a reward which Sarah is getting by performing good in class. It is a classic example of extrinsic motivation because a particular behavior is attained for a reward. On the other hand, Joe was performing well because he loves the material, it is a case of intrinsic motivation because the behavior is itself a reward.