总结：选用关系词的原则：
看关系词在定语从句中充当（什么成分）;关系词指代的是什么（人或物）。
Step3 关系词的特殊用法：
常见的几种关系词只用that 的情况：
(1) 当先行词为 all, everything , nothing, anything, little, much 等不定代词以及先行词被这些词修饰时。
All th at can be done has been done.
(2) 当先行词被形容词最高级或序数词修饰时。
This is the most beautiful park that I have visited.
(3) 当先行词被the only, the very, the right 修饰时。
This is the very book that I am looking for.
