I don't know about the video explanation for why C is wrong. C doesn't claim that all large nurseries do business primarily with commercial growers. A large nursery can sell mostly to commercial growers and still sell to most of the non-commercial growers if there aren't that many non-commercial growers. C claims that most non-commercial growers buy from non-large nurseries. The issue is that you can't take a contrapositive of a most claim. If these were non-intersecting sets, C would be correct.