source.list.d에 액세스할 수 없는 이유는 무엇입니까?

물음표는 모두 무엇을 위한 것인가요? 아래로 스크롤하여 이상한 출력을 확인하세요.

$ ls -la /etc/apt/sources.list.d/
ls: cannot access '/etc/apt/sources.list.d/additional-repositories.list': Permission denied
ls: cannot access '/etc/apt/sources.list.d/.': Permission denied
ls: cannot access '/etc/apt/sources.list.d/official-package-repositories.list': Permission denied
ls: cannot access '/etc/apt/sources.list.d/..': Permission denied
ls: cannot access '/etc/apt/sources.list.d/opera-stable.list': Permission denied
total 0
d????????? ? ? ? ?            ? .
d????????? ? ? ? ?            ? ..
-????????? ? ? ? ?            ? additional-repositories.list
-????????? ? ? ? ?            ? official-package-repositories.list
-????????? ? ? ? ?            ? opera-stable.list