如何安装R的readxl包?

我正在尝试安装 读XL

以下是RStudio输出:

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> library('readxl')
Error in library("readxl") : there is no package called ‘readxl’
> install.packages("readxl")
Warning in install.packages :
  unable to access index for repository http://cran.rstudio.com/src/contrib:
  cannot open URL 'http://cran.rstudio.com/src/contrib/PACKAGES'
Installing package into ‘C:/Users/Lenovo/Documents/R/win-library/3.5’
(as ‘lib’ is unspecified)
Warning in install.packages :
  unable to access index for repository http://cran.rstudio.com/src/contrib:
  cannot open URL 'http://cran.rstudio.com/src/contrib/PACKAGES'
Warning in install.packages :
  package ‘readxl’ is not available (for R version 3.5.0)
Warning in install.packages :
  unable to access index for repository http://cran.rstudio.com/bin/windows/contrib/3.5:
  cannot open URL 'http://cran.rstudio.com/bin/windows/contrib/3.5/PACKAGES'
> 

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> install.packages("readxl")
Installing package into ‘C:/Users/Lenovo/Documents/R/win-library/3.5’
(as ‘lib’ is unspecified)
--- Please select a CRAN mirror for use in this session ---
Warning: failed to download mirrors file (cannot open URL 'https://cran.r-project.org/CRAN_mirrors.csv'); using local file 'C:/PROGRA~1/R/R-35~1.0/doc/CRAN_mirrors.csv'
Warning: unable to access index for repository https://cloud.r-project.org/src/contrib:
  cannot open URL 'https://cloud.r-project.org/src/contrib/PACKAGES'
Warning: unable to access index for repository https://cloud.r-project.org/bin/windows/contrib/3.5:
  cannot open URL 'https://cloud.r-project.org/bin/windows/contrib/3.5/PACKAGES'
Warning messages:
1: In download.file(url, destfile = f, quiet = TRUE) :
  InternetOpenUrl failed: ''
2: package ‘readxl’ is not available (for R version 3.5.0) 
> 

我如何解决这个问题?