The Textbook Author Suggests That It Is Not a Good

The textbook author suggests that it is not a good idea to incorporate more than a few extra-credit points into students' grades.Why?

A) Incorporating extra-credit work into classroom grades typically affects only the grades of high-achieving students, so it is not worth the time and trouble.
B) Scores on extra-credit projects frequently suffer from low reliability.
C) The time spent on extra-credit assignments is better spent planning classroom activities.
D) Too many opportunities for extra-credit points allow students who have not met class objectives to get good grades.