We have three layers of skin ____ protect ourselves from diseases.

题目

We have three layers of skin ____ protect ourselves from diseases.
这个题目的答案是which/that,就是把skin当成先行词,后面变成一个定事从句.那这个空可不可以直接填to?

答案

We have three layers of skin 【to】 protect ourselves from diseases.
可以填to.
that / which 定语从句,句子主要强调skin.
to是做目的状语.句子强调的是目的：保护我们免受疾病侵害.
两个用法侧重点不同.但语法都对.