sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
    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-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-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-proposed main restricted universe multiverse
    deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed 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
    网络
    需要修改nameserver,于是执行sudo vi /etc/resolv.conf进行设置。
    将nameserver设置为8.8.8.8，如下图