According to the original text, we know that there is a negative correlation between crop yield and futures price. A good harvest will lower the price, while a poor harvest will raise it. This is the major premise.

It is said that the predicted abundant precipitation will meet the current crop growth needs, which means that the yield is guaranteed. This is a minor premise.

The conclusion of the original text is that the price of agricultural products will decrease.

C It is mentioned that the area outside the crop growing area has also received this precipitation, which has no effect on crop yield. It is mentioned that agricultural experts announced today that a disease that will reduce production will spread widely before crops mature. Obviously, D seriously weakens the correctness of the minor premise of the original text, that is to say, the output is not guaranteed and the price will not fall. Therefore, the emergence of the new premise D makes the argument of the original text unconvincing.