reuse it for the trusty image right now, too.

deb http://mirrors.cn99.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.cn99.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.cn99.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.cn99.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.cn99.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.cn99.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.cn99.com/ubuntu/ focal-proposed main restricted universe multiverse
deb-src http://mirrors.cn99.com/ubuntu/ focal-proposed main restricted universe multiverse
deb http://mirrors.cn99.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.cn99.com/ubuntu/ focal-backports main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu precise main universe
deb http://us.archive.ubuntu.com/ubuntu xenial main universe   
deb http://us.archive.ubuntu.com/ubuntu focal main universe


deb http://archive.ubuntu.com/ubuntu trusty restricted multiverse
deb http://archive.ubuntu.com/ubuntu trusty-security restricted multiverse
deb http://archive.ubuntu.com/ubuntu trusty-updates restricted multiverse
deb http://archive.ubuntu.com/ubuntu trusty-proposed restricted multiverse
deb http://archive.ubuntu.com/ubuntu trusty-backports restricted multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty universe
deb-src http://archive.ubuntu.com/ubuntu/ trusty universe
deb http://snapshot.debian.org/archive/debian/20070730T000000Z/ trusty main
deb-src http://snapshot.debian.org/archive/debian/20070730T000000Z/ trusty main
deb http://snapshot.debian.org/archive/debian-securit/20070730T000000Z/ trusty/updates main
deb http://ppa.launchpad.net/chromium-daily/stable/ubuntu trusty main
deb-src http://ppa.launchpad.net/chromium-daily/stable/ubuntu trusty main