正确答案是B（for dozens of years）。A选项dozen应用复数；C选项year应用复数；D选项应去掉of。其实有不少同学对dozen和score这两个词颇感到头疼，解答如下：

1.作数量词时，dozen十二个，score二十个。用法与hundred, thousand …等类似。如：

a dozen apples 十二个苹果(一打苹果)，

two score chairs四十张椅子，

three hundred people三百个人。

2.作单位词时，score (二十) 后可接of，但dozen (一打)后接of很罕见。如：

five score (of) fish一百尾鱼(省略of则score为数量词)，

three dozen eggs 三打鸡蛋

1992年高考（全国卷）第29小题：Shortly after the accident, two _____ police were sent to the spot to keep order. (A. dozen of  B. dozens  C. dozen  D. dozens of) 

3.在表“几十个”不确定的泛指数时用dozens of或scores of，用法同tens of, hundreds of, thousands of…。如：

dozens of / scores of students好几十个学生（所以，你问题中的这道题选项B是正确的）

4.dozens of和scores of前若用some, several, a few, many等词时，dozen和score可去掉“s”(很少保留)，同时也去掉of。如：

several dozens/scores of people = several dozen/score people

5.当后面有人称代词(you, us, them)时，名词前有定冠词(the)、物主代词(my, your, his, her, their, its)以及指示代词(these, those)限定时，要用“数词+dozen/score +of”，dozen和scores均不加“s”。如：

a dozen of them, 

two score of the books, 

three score of my sheep, 

four dozen of those children

6.有三个惯用表达需特别记忆：

three score (years) and ten 七十岁，古稀之年；

three score of people 六十人；

three score and ten people七十人