21. You __________ feel all the training a waste of time, but I’m a hundred percent sure later you’ll be grateful you did it.
A
should
B
need
C
shall
D
may
should应该；need需要；shall将会；may（不确定）可能。故选项D符合答案。
根据句意：你可能觉得所有的训练都是浪费时间，但是我有100%的把握你以后会感激它的。
常见情态动词含义记忆不准致错。