The variation in the numbers of phage resistant mutants (among different E. coli cultures) seen in the Luria-Delbruck fluctuation test is a reflection of: Group of answer choices Differing levels of mutagenesis in the different cultures The differing media in which the bacterial cultures were grown The concentration of phages in the phage suspension added to the cultures The growth phase of the bacterial cultures, before being challenged with phage When the mutation(s) conferring resistance arose (i. e. early or late), during growth of the different cultures