replace the original content with following source list
12345678910
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise main restricted universe multiverse
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise-security main restricted universe multiverse
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise-updates main restricted universe multiverse
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise-backports main restricted universe multiverse
deb http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise-proposed main restricted universe multiverse
deb-src http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise main restricted universe multiverse
deb-src http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise-security main restricted universe multiverse
deb-src http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise-updates main restricted universe multiverse
deb-src http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise-backports main restricted universe multiverse
deb-src http://ftp.cuhk.edu.hk/pub/Linux/ubuntu precise-proposed main restricted universe