It's about ___the thickness of a human hair

12. It's about__ , the thickness of a human hair.

A. two-fifteenth B. two-fifteenths C. two fifteen D. two fifteens

12.答案为B。

分数做定语，分子用基数词，分母为序数词，当分子大于1时，分母用复数，

本题中，其它表达方式均不正确。