冠词的使用She was

冠词的使用
She was ___ founder of ___ order of nuns called the Missionaries of Charity.
A.a,an B.a,/ C.the,/ D.the,an
（D)
A 和D有什么区别,为何不选A．

首先 the是特指,a,an是泛指
an修饰order指好多人
the特指,实际指the Missionaries of Charity
语言表达不好,见谅!