The above arguments will lead to higher grain output and lower grain prices.

Option a refutes the argument and has a weakening effect.

Option b conforms to the argument and cannot be weakened or has little impact.

Serious pests appear in the mature period of option C, which will greatly reduce production and lead to an increase in food prices. The weakening effect is obvious.

Option d is only a comparison of other commodities, which has little effect on the grain itself. There is no weakening effect.

Option e says that drought is not the most serious threat, which proves that even if there is drought, it will not greatly reduce production. The weakening effect is very poor.

To sum up, the most weakened option is C.