帮忙看看这句话该怎么弄对
You can only be sure of ______ you have at present; you cannot be sure of something _____ you might get in the future.
我认为前半句缺少宾语,可以填入which.
后半句什么都不缺,或者也可以说缺宾语,可以用which或者that.
但是答案是前者填入what,后者填入that(省略）
什么时候该用what呢?