什么是active security clearance？怎么才能弄到这个？

还有啊，看到tax term有这些选项，除了第一个不知道剩下的都有什么区别？

contract, corp-to-corp

contract, independent

contract, w2

contract to hire, corp-to-corp

contract to hire, independent

contract to hire, w2

谢谢啦。