During a particularly dry growing season in a southern state, farmers noticed that there is a delicate balance between the number of seeds that are planted per square foot and the yield of the crop in pounds per square foot. The yields were the smallest when the number of seeds per square foot was either very small or very large. What is the explanatory variable for this relationship?

yield of the crop
location of the farm
precipitation for the growing season
number of seeds planted per square foot

I think it's (D).
number of seeds planted per sf