Ubuntu 17.04- Package has no installation candidate解决方式

Ubuntu 17.04 安装完虚拟机之后，如果想装ssh软件进行连接，发现几乎所有的包都装不成功。

root@zengxijin-va:/home/zengxijin# apt-get install openssh-server
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package openssh-server 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 'openssh-server' has no installation candidate

报的报错都是Package ‘xxx’ has no installation candidate。
解决办法：
（1）备份/etc/apt/sources.list

sudo cp -i /etc/apt/sources.list /etc/apt/sources.list.backup</