When using two kinds of phage to do hybridization, such as a+b+× a-b-, which of the following formula can be used to calculate the recombinant frequency of a and b?
A.a+b-/(a+b++a+b-)
B.a-b+/(a+b++a-b+)
C.(a+b-+a-b+)/(a+b-+a-b++a+b+)
D.(a+b-+a-b+)/(a+b-+a-b++a+b++a-b-)