Two Different Strains of a Mutant Phage Infected a Single

Two different strains of a mutant phage infected a single bacterium.One phage strain is a^-b⁺ and the other is a⁺b^-.The coinfected phages produced the wild-type phenotype in the bacterium: progeny phages were diluted and plated on a bacterial lawn.The resulting plaques produced the following genotypes: 19 a⁺ b⁺, 31 a^-b⁺, 29 a⁺b^-, 21 a^-b^-.What is the recombination frequency between the a and b genes?

A) 10%
B) 20%
C) 30%
D) 40%
E) 50%