【R语言编程】---install.packages命令报错解决

不知道哪里出了问题，

Warning in install.packages :
  unable to access index for repository https://mirrors.ustc.edu.cn/CRAN/src/contrib:
  cannot open URL 'https://mirrors.ustc.edu.cn/CRAN/src/contrib/PACKAGES'
Warning in install.packages :
  unable to access index for repository https://mirrors.ustc.edu.cn/CRAN/src/contrib:
  cannot open URL 'https://mirrors.ustc.edu.cn/CRAN/src/contrib/PACKAGES'
Warning in install.packages :
  package ‘ggtree’ is not available (for R version 3.5.1)
Warning in install.packages :
  unable to access index for repository https://mirrors.ustc.edu.cn/CRAN/bin/windows/contrib/3.5:
  cannot open URL 'https://mirrors.ustc.edu.cn/CRAN/bin/windows/contrib/3.5/PACKAGES'

解决方案

options(download.file.method = 'libcurl',url.method='libcurl')

ps:如果关掉R重启，又会报以前的错，只能再次输入这个命令，还找不到永久解决的办法