When did harvesting crops by machine become possible?

A. Farming equipment was improved to harvest crops quicker while using less fuel, allowing for machine harvesting.

B. Farmers developed new techniques, such as crop rotation and deep plowing, which made machine harvesting possible.

C. Farmers used agricultural research to develop plants that grew faster and matured at the same time, making machine harvesting possible.

D. Farmers graded or leveled the land to the extent that machine harvesting of crops was made possible.