When performing in vitro mutagenesis, a vector is used that may contain a positive marker (e. g., a gene for resistance against the antibiotic neomycin) and a negative marker, such as the thymidine kinase gene. cells that carry the latter will die when exposed to a drug called ganciclovir. if a researcher exposed stem cells to a targeting vector with these two markers, and subsequently treated them with neomycin and then ganciclovir, which would be true of the surviving cells? a. any surviving cells will have been correctly targeted (i. e., the gene of interest will have been removed).b. half of the surviving cells will still contain the gene targeted for inactivation, whereas in the remaining half this gene will have been removed. c. any surviving cells will not carry either the positive or negative markers. d. any surviving cells will still contain the original, targeted gene (i. e., these cells could not be used to generate knockout mice)