Based on Prior Experiences by the USDA, the Farm Extension

Based on prior experiences by the USDA, the farm extension advisor did what correctly in his efforts to have farmers adopt the new hybrid variety of corn?

A) He grew a demonstration plot of the new hybrid corn.
B) He brought in a supply of fertilizer at the time of planting.
C) He rented a farm and lived among the farmers.
D) He conducted free classes on the proper way to cultivate and plant.