To apply for a position, please send us your resume, along with a letter ______
your sales plan.
A : will describe
B : describes
C : describing
D : described
答案为C ，我本来选B.
想请问是何处看出要用ing呢？

原本是 which describes，省略which后动词变现在分词。

... a letter which describes your ... 或如C选项以现在分词修饰