Assuming One Could Identify a "Bad" Trait, Why Might It

Assuming one could identify a "bad" trait, why might it be impossible to eliminate it by eliminating an associated "bad" gene by germ-line gene therapy?

A) It's impossible to isolate a single gene.
B) The gene eliminated might do other important things, often more than one gene controls a trait, and the environment might matter, too.
C) Sterilization is the only way to prevent a gene getting passed along; it eliminates a whole lot of genes, not just a "bad" one.
D) The environment is always going to influence the genes, so removing a single "bad" one without changing the environment, might not help.
E) There is, as of now, no known correlation between any specific gene and particular illness or disorder.