我无法mc在 Ubuntu 18.04.1 上安装 Midnight Commander ( )。
/etc/apt/sources.list 文件：
deb http://archive.ubuntu.com/ubuntu bionic main
deb http://archive.ubuntu.com/ubuntu bionic-security main
deb http://archive.ubuntu.com/ubuntu bionic-updates main
这是我尝试安装时得到的答复：
$ sudo apt-get install mc
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package mc 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
这没有帮助（#1）：
$ sudo apt-get update
这没有帮助（#2）：
$ sudo add-apt-repository …