Steve Is a Professional Basketball Player

Multiple Choice

Steve is a professional basketball player.He never knows for sure which of his shots will result in a basket,but the more shots he takes,the more baskets he makes.How is Steve's shooting reinforced in this example

A)  on a variable-ratio schedule 
B)  on a fixed-interval schedule 
C)  on a variable-interval schedule 
D)  on a fixed-ratio schedule