Riddle #439

Question: What word doesn't belong in this group? That, hat, what, mat, cat, sat, pat, or chat?

Question: What do you call a sad bird?

Question: What kinds of balls do dragons play soccer with?

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?