the students in this school is about four thousand of them are boysA. A number of three

问题补充：

the students in this school is about four thousand,of them are boys．

A. A number of, three fifth

B. The number of, three fifths

C. A number of, three fifths

D. The number of, three fifth

答案：

A. A number of, three fifth

(答案→)B. The number of, three fifths

C. A number of, three fifths

D. The number of, three fifth

解析：此题考查分数的表示方法。分数在英语中通常是借助于基数词和序数词来共同表达的。其中基数词表示分子，序数词表示分母。分子超过1，分母要加s.分数修饰名词时，若该名词是不可数名词只能用单数；若是可数名词，用单数或复数均可。但是，若它们在句子中作主语，则谓语动词是用单数还是复数取决于名词，即与分数所修饰的名词保持一致。根据语境可知此句的含义是这所学校的学生大约有四百人，其中五分之三的是男生。表示…的数量要用名词短语the number of，五分之三要用three fifths, 故选B。