In her letters she asked them to decide a day for ____mothers

题目

In her letters she asked them to decide a day for ____mothers
A.both B.all C,every D.only her 我一直纠结于选B还是选C,不知道选哪一个,选那个的理由是什么.

答案

mothers是复数形式,所以肯定不能用C了,every是要加名词单数的.
all是全部的意思,加名词复数