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。