I still really don't understand why (A) is so wrong. I understand why (B) is better.
Context:
Drew received roses with no card.
Premises:
1) If knew drew->knew preferred violets
2) If did not know drew->would have sent a card
Conclusion:
1) The florist made a mistake (wrong flowers, forgot the card, not meant for drew)
(A) says that most people send roses. If most people send roses, isn't that an alternative explanation for why Drew got roses? I took it as, most people send roses just out of habit. Like when choosing flowers, they usually pick roses because of whatever reason. But maybe I should have taken it to mean that most people send roses for a reason? I guess this answer would require too many assumptions...