新概念英语第三册第五课

However ,he had at last been allowed to send a fax in which he infoemed the editor that he had been arrested.....

然而，最后他被允许发送一份传真，其间他告知编辑说，他已经被捕。
in which 中的which = fax

这句也可以这么说
However ,he had at last been allowed to send a fax ，in the fax, he infoemed the editor that he had been arrested....

这样，你品品，还不理解，我再来补充解释。