请问sth is sufficient to do sth结构中的不定式作什么成分?该结构中的主语和不定式构成什么关系?学生查了牛津,朗文,科斯林例句均为主谓关系,The reasons aren't sufficient to justify the ban. The money aren't sufficient to cover everything that needs doing. One metre of fabric is sufficient to cover the exterior of a 18-in-diameter hatbox.