Riddle #546

Question: I'm not a pool, a tree, a slave, a website, or a shirt.

What am I?

Question: What do you call a sad bird?

Question: What has a bed that you can't sleep in?

Question: How are doughnuts and golf alike?

Question: What can be broken but never forgotten?

Question: People see through me, but none pass through without killing me. I can be anywhere, and can be seen double or single. What am I?