Fractions of a Field of Bt Crops Planted with Non-Bt-Expressing

Fractions of a field of Bt crops planted with non-Bt-expressing varieties:

A) ensure that herbivore genes conferring susceptability to Bt are well represented in the next generation.
B) ensure that selection for Bt resistance is not too intense within herbivore populations.
C) impede the complete eradication of important agricultural pests.
D) lead to higher losses to pests in the short term than if no fractions were planted with non-Bt-expressing varieties, but maintain the effectiveness of Bt in protecting crops in the long term.