우분투 7.04 build-essential 설치 오류

우분투 7.04를 설치했는데..
build-essential을 설치하려고 하니..
계속 아래와 같은 에러가 뜹니다.. ㅠ,ㅠ
어찌해야 하는지요??

root@dragon:~# apt-get install build-essential
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package build-essential is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package build-essential has no installation candidate
root@dragon:~#