无法在rstudio上下载软件包
以下代码返回以下回复
> options(download.file.method="libcurl")
WARNING: You are configured to use the CRAN mirror at https://cran.rstudio.com/. This mirror supports secure (HTTPS) downloads however your system is unable to communicate securely with the server (possibly due to out of date certificate files on your system). Falling back to using insecure URL for this mirror.
此外，此代码还返回以下消息：
> options(repos = c(CRAN = "http://cloud.r-project.org")); available.packages()
Warning: unable to access index for repository http://cloud.r-project.org/src/contrib:
cannot open URL 'http://cloud.r-project.org/src/contrib/PACKAGES'
Package Version Priority Depends Imports LinkingTo Suggests Enhances License License_is_FOSS
License_restricts_use OS_type Archs MD5sum NeedsCompilation File Repository
有人可以帮忙吗？