Which one of the following was NOT a reason for the emergence of the single-family peasant farm as the basic unit of agricultural production in the Early Middle Ages? Select one: a. The mounted soldier made warfare expensive, so many warriors became full-time farmers. b. The declining birthrate made it necessary for entire families to work in agricultural production. c. Horses made it easier, although more expensive, for farmers to support themselves. d. It was more economically feasible for landowners to rent their land to farmers. e. Villa owners could no longer purchase slaves.

It's true that those peasants tend to make the entire families to work in agricultural production, but it has nothing to do with the decline of birth rate.

Since at that time farming technologies haven't been invented, These poor farmers realize that having more children equals having more labor in the field. So they plan ahead and make as much children as possible and increase their total production.