25.A group of genetics students mix two auxotrophic strains of bacteria: one is leu⁺ trp⁺ his⁻ met⁻ and the other is leu⁻ trp⁻ his⁺ met⁺. After mixing the two strains, they plate the bacteria on minimal medium and observe a few prototrophic colonies (leu⁺ trp⁺ his⁺ met⁺). They assume that some gene transfer has taken place between the two strains. How can they determine whether the transfer of genes is due to conjugation, transduction, or transformation?