这句为什么用had better 而不用would rather?

题目

这句为什么用had better 而不用would rather?
If Jack is as sick as you say,she________ go home.

答案

这句话字面意思是：如果JACK像你所说的一样生病了,她最好回家.
would rather 是宁可、宁愿
had better是最好