779.Doctors say that exercise is important for health,but it ____ be reguolar exercise.
A.can
B.will
C.must
D.may
780.Just be patient.You ____ expect the world to change so soon.
A.can't
B.needn't
C.may not
D.whether
779.答案是C;can一般表示“可能或能力”；will表示愿意；may表示许可；must表示必须。
780.答案是A;can否定形式can't可以表示推测，根据题意选A。
