如何区分if whether that 用法,

题目

如何区分if whether that 用法,
we'd better check the number again and make sure ____ everybody is here.
a.that b.if c.when d.whether
应该选哪一个 ,

答案

选A.
whether和that最大的区别就在于whether后面跟的是疑问语气的句子,
而that跟的是肯定语气的句子.
你这句话时肯定语气所以用that.