An experiment was conducted in which his-trp-leu+ mutants were crossed with his+trp+ and leu- mutants.

An experiment was conducted in which his-trp-leu+ mutants were crossed with his+trp+ and leu- mutants.

However, no recombinants occurred when a filter was placed in the U-shaped Davis tube to separate the strains.  Why was that?

a.    they were only plated on minimal media, instead of on nutrient agar first

b.    they needed conjugation to occur

c.    they needed transduction to occur

d.    they needed transformation to occur

e.    they needed lygogeneis to occur