Gene Knockouts Often Result in Embryonic Lethality

Gene knockouts often result in embryonic lethality. What strategy do investigators often use to make it possible to study a gene's role later in development?

A) Gene knockout using an inducible tetracycline resistance gene.
B) Use of embryonic stem cells from a strain of mice with brown fur and a blastocyst from astrain of mice with black fur.
C) Implantation of embryos into a pseudopregnant female.
D) Inducible Cre/Lox system for site-specific recombination.