清华大学镜像源：
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
阿里云镜像源：
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
中科大镜像源：
deb http://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
将 focal替换为你使用的Ubuntu版本代号，例如 bionic、xenial等。
保存并关闭文件：按下 Ctrl + X，然后按 Y确认保存，最后按 Enter键退出编辑器。