在 Ubuntu 15.04 中,执行 sudo apt-get update 后出现此信息:
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/vivid/main/binary-amd64/Packages 404 Not Found
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/vivid/main/binary-i386/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
请访问此链接: http://extras.ubuntu.com/ubuntu/dists/
正如它所说,vivid 不存在。他们移动它了吗?他们会把它移回去吗?我该如何修复?