Some students think it cool to have their hair dyed yellow.为什么用it 不用that 高三狗求指教

题目

Some students think it cool to have their hair dyed yellow.为什么用it 不用that 高三狗求指教

答案

一般在一个句子中（前后句中间有标点的不算）,it 较多用作代词,that较多用于引导从句.
这里 it 用于指代 to have their hair dyed yellow 这件事