What is the main idea of the passage（）

A. To stand down the fairway at a golf course.
B. To spot the balls as they landed so the golfers could find them.
C. To drive the oxen that plowed the cane fields.
D. To watch the sugar-cane plantation.

A. the choice between spiritual encouragement and monetary rewards.
B. the amount of monetary rewards for student’ creativity.
C. the study of relationship between actions and their consequences.
D. the effects of external rewards on student’s performance.