上面的错误是什么意思？什么是安装候选？
我试图做
sudo apt-get install munin-memcached
我收到这条消息：
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package munin-memcached 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 munin-memcached has no installation candidate
我用谷歌搜索了这个问题，有人说要做一个，apt-get upgrade但它仍然没有解决我的问题。
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 …