These blue pens are Cindy's,and those red pens are ---------

These blue pens are Cindy's,and those red pens are _【hers】__,too

Does Mary have a small mouth?
No,she has a wide __【one】.
再问： 为什么是one呢？两句mouth都是指的一个啊，老师说同类用one，同一个用it
再答： 一般是用a+形容词+one 来代指事物，it没有这种用法