after that 与 after which 的用法比较

题目：You should open an account at the online bank first, after _______ you can buy anything online. Let's check it out online.

答案：which

我想问的是 after 能不能当做连词看，如果能，是不是可以填 that？如果不能，请老师给以指导，谢谢！