get through 与 get across 都有“通过”的意思,请问它们有什么区别呢

题目

get through 与 get across 都有“通过”的意思,请问它们有什么区别呢
有这么一道题：
There were a lot of people standing at the door and the small girl couldn't get _____.
答案是through ,我想问为什么不可以是across

答案

get through在用在具体的事物上时,是通过一个入口,一扇门,窗等待,比如get through the door
get across是指通过一片地域,领域等等,如get across the great desert.穿过广大的沙漠.
当二者用语抽象的事物时,都可以指事某人明白某事.get through 还可以指通过一段艰难的历程,经历过困难.如I finally got through the disease.终于我战胜了病魔.