我尝试在 trusty/Ubuntu 14.04 中安装 ffmpeg 并收到以下消息：
$sudo apt-get install ffmpeg
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package ffmpeg 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 'ffmpeg' has no installation candidate
为什么包不可用？