IMO B
The reason soyabean planters planned to enter cotton business started from the motivation that prices of cotton have been rising.
The price rise of cotton may happen because of any of these causes:
1. The orginal problem of insect infestation caused the price rise.
2. Alternate cause , increase in demand ,caused price rise.
In this particular case , cause 1 seems to be more probable because the premise says this as the motivation behind soybean planters' decision to move to cotton farming . So negating that would weaken more.
The main assumption that the price would continue to remain high is most weakened when the cause due to which the prices of cotton were high does not remain. i.e when a new pesticide is available and that can rectify the problem. This would lead the prices to come down again.
Demand seems to be out of scope in this case.
Please correct me if anyone feels its not making sense.