2.What do you mean,there are only ten tickets?There __ ___ be twelve.

题目

2.What do you mean,there are only ten tickets?There __ ___ be twelve.
帮我解释下情态动词shall 和 should 的不同用法 can 和must

答案

ought to 与 should 差不多,应该,是此题答案
must一定,can用否定的,can't不可能
shall在这类的情态动词里不常见,