I think she is sure to come if she ______free.A will be B is能解释选择答案的原因吗

题目

I think she is sure to come if she ______free.A will be B is能解释选择答案的原因吗

答案

B is
楼主 选B be sure to come确定回来 已经表示将来了 后边用现在时就可以了 相当于
I think she will surely come if she is free