sudo apt update

Hit:1 http://mirrors.ustc.edu.cn/ubuntu xenial InRelease
Get:2 http://mirrors.ustc.edu.cn/ubuntu xenial-updates InRelease [94.5 kB]
Get:3 http://mirrors.ustc.edu.cn/ubuntu xenial-backports InRelease [92.2 kB]
Get:4 http://mirrors.ustc.edu.cn/ubuntu xenial-security InRelease [94.5 kB]
Hit:16 http://ppa.launchpad.net/adamreichold/qpdfview-dailydeb/ubuntu xenial InRelease
Get:5 http://mirror.bit.edu.cn/ubuntu xenial-updates/main amd64 Packages [244 kB]   
Hit:18 http://ppa.launchpad.net/transmissionbt/ppa/ubuntu xenial InRelease
Hit:19 http://ppa.launchpad.net/webupd8team/atom/ubuntu xenial InRelease           
Get:6 http://mirror.bit.edu.cn/ubuntu xenial-updates/main i386 Packages [240 kB]
Get:7 http://mirror.bit.edu.cn/ubuntu xenial-updates/main Translation-en [96.5 kB]
Get:8 http://mirror.bit.edu.cn/ubuntu xenial-updates/universe amd64 Packages [137 kB]
Get:9 http://mirror.bit.edu.cn/ubuntu xenial-updates/universe i386 Packages [134 kB]
Get:10 http://mirror.bit.edu.cn/ubuntu xenial-updates/universe Translation-en [68.5 kB]
Get:11 http://mirror.bit.edu.cn/ubuntu xenial-security/main amd64 Packages [116 kB]
Get:12 http://mirror.bit.edu.cn/ubuntu xenial-security/main i386 Packages [112 kB]
Get:13 http://mirror.bit.edu.cn/ubuntu xenial-security/main Translation-en [44.1 kB]
Get:14 http://mirror.bit.edu.cn/ubuntu xenial-security/universe amd64 Packages [33.1 kB]
Get:15 http://mirror.bit.edu.cn/ubuntu xenial-security/universe i386 Packages [33.0 kB]
Get:17 http://mirror.bit.edu.cn/ubuntu xenial-security/universe Translation-en [19.9 kB]
Fetched 1,559 kB in 6s (249 kB/s)                                                   
Reading package lists... Done
Building dependency tree       
Reading state information... Done

cat sources.list

# deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-updates main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## universe WILL NOT receive any review or updates from the Ubuntu security
## team.
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial universe
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-updates universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-updates multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu xenial partner
# deb-src http://archive.canonical.com/ubuntu xenial partner

deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-security main restricted
# deb-src http://security.ubuntu.com/ubuntu xenial-security main restricted
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-security universe
# deb-src http://security.ubuntu.com/ubuntu xenial-security universe
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-security multiverse
# deb-src http://security.ubuntu.com/ubuntu xenial-security multiverse