时间状语从句as,while,when怎么区别
____ (As) they were enjoying exploring,the crowd of people became larger and larger.
 第一题为什么用as,不用while / when?
_______(While / When)the crowd was pushing Darren in all directions,someone stepped on his feet.
 第二题为什么可以用while/when,但是不能用as?
_______(When)  Darren finally pushed his way out,he couldn't find his two friends.
什么用when?为什么不用as?
 
请针对我提出的三个具体问题给出答案，而不是复制、粘贴一些语法的解释，