When a Farmer Grows Cotton Containing the Gene for Bt

When a farmer grows cotton containing the gene for Bt toxin,they often leave a strip of non-Bt cotton around the outside of the field for what reason?

A) because the non-Bt cotton grows better
B) because the non-Bt cotton does not drive the moths and caterpillars to extinction
C) to allow the non-Bt and Bt cotton plants to cross pollinate
D) to allow some rr bollworm caterpillars to survive
E) to allow some RR bollworm caterpillars to survive and limit selection pressure for resistance