Two Mutations That Affect Plaque Morphology in a Particular Phage

Two mutations that affect plaque morphology in a particular phage (a^- and b^-) have been isolated. Phages carrying both mutations (a^-b^-) are mixed with wild-type phage (a⁺b⁺) and added to a culture of bacterial cells. Following infection and lysis, samples of the phage lysate are collected and cultured on bacterial cells. The following numbers of plaques are observed: $$\begin{array}{l}\begin{array} { l r } \text { Plaque Phenotype}&{ Number }\\a ^ { + } b ^ { + } & 2043 \\a ^ { + } b & 320 \\a ^ { - } b ^ { + } & 357 \\a^- b ^ { - } & 2134\end{array}\end{array}$$ What is the frequency of recombination between the a and b genes?

A) 7%
B) 14%
C) 42%
D) 28%
E) 36%