Why has membership in labor unions decreased

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.