单选题 The cultured E. coli system was infected by two types of bacteriophages. One type is a^-b^-, another is a⁺b⁺. Then the lysate was sampled and smeared on medium. The data of the bacteriophage growth by counting plaques is as follows: a⁺b⁺=4750, a⁺b^-=370, a^-b⁺=330, a^-b^-=4550. How much is the recombination rate between a and b? ______
A．70% B．12.8% C．7% D．15%