Which Statement Refers to a Variable Reinforcement Schedule

Multiple Choice

Which statement refers to a variable reinforcement schedule?

A) "Sometimes I get As when I study and sometimes I don't. So if I happen not to get an A on this test, I know I'll have to keep studying and eventually I will succeed."
B) "If I don't get an A on this test, I will give up."
C) "If I get an A on this test, I'm a genius."
D) "I love teachers who give mainly As."