McKenna Had an Unplanned Pregnancy When She Was 19 Years

McKenna had an unplanned pregnancy when she was 19 years old.Following her pregnancy,McKenna started taking a birth control pill each day to prevent another pregnancy.What does this example illustrate

A)  Preventing a pregnancy acts as a positive reinforcer for taking birth control pills. 
B)  Preventing a pregnancy acts as a negative reinforcer for taking birth control pills. 
C)  Taking birth control pills acts as a negative reinforcer for preventing a pregnancy. 
D)  Taking birth control pills acts as a positive reinforcer for preventing a pregnancy.