liunx安装ssh遇到的问题。

在Ubuntu上安装SSH。输入命令：

$ sudo apt-get install ssh
出现以下的错误：

Reading package lists... Done
Building dependency tree

Reading state information... Done
Package ssh 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
However the following packages replace it:
openssh-client ssh-askpass-gnome
E: Package 'ssh' has no installation candidate

该怎样解决？求大神指导。。。