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
答案為C ，我本來選B.
想請問是何處看出要用ing呢？

原本是 which describes，省略which後動詞變現在分詞。

... a letter which describes your ... 或如C選項以現在分詞修飾