must not /do not have to / need not to有什么差别 ? 单选题的时候让人很纠结啊

(1) must not = prohibition (it is not allowed)
(a) The teams[ must not] have more than 11 players on the field during a match.
(b) Players [must not] wear anything which might injure another player.
(2) need not = option (it is not necessary) = do not have to
(a) But there [need not] even be 11 players playing for each team.
(b) Players [need not/do not have to] play the ball with their feet only.