为什么我无法访问sources.list.d？

Question

所有的问号是做什么用的？向下滚动查看奇怪的输出。

$ 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