apt-get install build-essential 失败并出现以下错误：
~$ sudo 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
sudo apt-get update在尝试此安装之前，我刚刚运行。
文件夹 /etc/apt/sources.list.d 是空的。这是 /etc/apt/sources.list 的主要内容：
$ grep deb\ http: /etc/apt/sources.list #(with "deb http://" deleted)
us.archive.ubuntu.com/ubuntu/ raring main restricted
us.archive.ubuntu.com/ubuntu/ …apt ×1