Tanner wants to fill his swimming pool using two hoses, each of which sprays water at a constant rate. Hose A fills the pool in a hours when used by itself, where a is a positive integer. Hose B fills the pool in b hours when used by itself, where b is a positive integer. When used together, Hose A and Hose B fill the pool in 6 hours. How many different possible values are there for a?
Answer Choices
A. 5
B. 6
C. 9
D. 10
E. 12