———Are you willing to join us tomorrow?

题目

———Are you willing to join us tomorrow?
----- If ______,I will give you a phone call ahead of time.
A.will be B.am C.will do D.do
我认为是B,可是答案是A,为什么呢?

答案

will表示 愿意.是情态动词,可用于条件状语从句.
EG:If you will go,please tell me .