Answer:
Step-by-step explanation:
The correct answer is OD. The farm's marginal cost for fertilizer will decrease.
This is because even though the new fertilizer costs 20% more to produce, its better results lead to a 25% increase in crop production. This means that the cost of producing each additional unit of crop is now lower due to the improved fertilizer. As a result, the marginal cost of using the new fertilizer will be lower than the marginal cost of using the old fertilizer.