Answer: From the comprehension , we can state the following two arguments:
(a.) If a poor corn harvest is predicted, prices of corn futures rise.
(b.) If a bountiful corn harvest is predicted, prices of corn futures fall.
Also, it has been predicted that much-needed rain for the corn-growing region will start tomorrow.
Now, if the following is true, then it weakens the above argument: Agriculture experts announced today that a disease that has devastated some of the corn crop will spread widely before the end of the growing season.
Since, it has already been predicted that it'll rain in requirements with the growth of the crop but if the crops are destroyed before the growing season , then there will be a decrease in cultivation and thus the prices will rise.
Therefore, the correct option is (D)