A Farmer Uses Triazine Herbicide to Control Pigweed in His

A farmer uses triazine herbicide to control pigweed in his field. For the first few years, the triazine works well and almost all the pigweed dies; but after several years, the farmer sees more and more pigweed. Which of these explanations best describes this observation?

A) Triazine- resistant pigweed has less- efficient photosynthesis metabolism.
B) Increased rainfall due to climate change washes away the herbicide more often, so the pigweed has reduced exposure to the triazine.
C) The pigweed mutated in response to the herbicide, creating a new triazine- resistant species.
D) Only triazine- resistant weeds survived and reproduced, so each year more pigweed was triazine- resistant.