utils:::menuInstallPkgs()
--- Please select a CRAN mirror for use in this session ---
Warning: unable to access index for repository http://mirrors.ustc.edu.cn/CRAN/bin/windows/contrib/3.1
Warning: unable to access index for repository http://www.stats.ox.ac.uk/pub/RWin/bin/windows/contrib/3.1
Error in install.packages(NULL, .libPaths()[1L], dependencies = NA, type = type) :
  no packages were specified
In addition: Warning message:
In open.connection(con, "r") : cannot open: HTTP status was '404 Not Found'
>
这是什么原因？为什么显示没有这种程序包?初学者，各位大神指点迷津