The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype. Experiment A Experiment B Experiment C phe⁻ ala⁻ 26% leu⁻ ala⁻ 65% leu⁻ phe⁻ 71% phe⁺ ala⁻ 50% leu⁺ ala⁻. 48% leu⁺ phe⁻ 21% phe⁻ ala⁺ 19% leu⁻ ala⁺   0%   leu⁻ phe⁺ 0% phe⁺ ala⁺   3% leu⁺ ala⁺   4% leu⁺ phe⁺   3% In Experiment B, why are there no transductants with the genotype leu⁻ ala⁺?