How Is the Difficulty Level of a Set Goal Related

How is the difficulty level of a set goal related to success in meeting the goal?

A) If the goal is too easy, the athlete does not push themselves to excel; if the goal is too challenging, the athlete can become discouraged and withdraw effort
B) More difficult goals tend to be objective and more easily measured
C) Goal difficulty is correlated with effort; the more challenging the goal, the more effort an athlete will put forth, and the more likely they are to succeed
D) Goal difficulty is unrelated to success in meeting the goal