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, no matter how often he applies triazine. Which of these actions is most likely to solve the farmer's problem?

A) adding triazine more often to his fields
B) trying a different herbicide
C) applying the triazine to the soil and roots instead of spraying it over the leaves
D) increasing the amount of triazine he puts on his fields