Once DNA Has Been Isolated and Cloned, Why Can't It

Once DNA has been isolated and cloned, why can't it just be physically inserted into a cell and become functional?

A) Doing that would damage the cell membrane and kill the cell
B) It's impossible to pick up a single gene.
C) A gene floating around in cytoplasm would get too clogged to work properly.
D) This is theoretically possible, but the techniques and tools we have now simply aren't good enough.
E) The organism's enzymes would break down the foreign DNA as if it were an infection.