the mechanism behind the idea of propelling an object into the air, has been around for well over two thousand years. 1、这句里面的behind怎么解释，做什么成分？ 2、around 可以理解成about吗... 展开
the mechanism behind the idea of propelling an object into the air, has been around for well over two thousand years. 1、这句里面的behind怎么解释，做什么成分？ 2、around 可以理解成about吗？ 3、well over 怎么解释？ 里面还有哪些要注意的语法点，也顺便说下，谢谢！ （初学者问） 收起
