are
句子的主语是One third of the students =20人,是复数意义的；be根据主语的单、复数而变换形式主语是one third还是the students ，be动词时根据one third还是the students 来定的？主语是one third，其中的of the students 是one的定语。be动词时根据one third定，但同时要考虑到后面的定语的名词是复数名词，还是不可数名词。又如；One third of the water is......