解决R安装包时，提示“无法与服务器建立链接“的错误。
解决方法： 修改镜像目录后，就可以了
> install.packages("xml2")
Installing package into ‘C:/Users/ytwang/Documents/R/win-library/4.0’
(as ‘lib’ is unspecified)
trying URL 'https://cran.rstudio.com/bin/windows/contrib/4.0/xml2_1.3.2.zip'
Warning in install.packages :InternetOpenUrl failed: '无法与服务器建立连接'
Error in download.file(url, destfile, method, mode = "wb", ...) : cannot open URL 'https://cran.rstudio.com/bin/windows/contrib/4.0/xml2_1.3.2.zip'
Warning in install.packages :download of package ‘xml2’ failed
> options(BioC_mirror="https://mirrors.tuna.tsinghua.edu.cn/bioconductor")
> install.packages("xml2")
Installing package into ‘C:/Users/ytwang/Documents/R/win-library/4.0’
(as ‘lib’ is unspecified)
trying URL 'https://cran.rstudio.com/bin/windows/contrib/4.0/xml2_1.3.2.zip'
Content type 'application/zip' length 3004192 bytes (2.9 MB)
downloaded 2.9 MBpackage ‘xml2’ successfully unpacked and MD5 sums checkedThe downloaded binary packages are inC:\Users\ytwang\AppData\Local\Temp\RtmpGgtIDv\downloaded_packages