-
I'd say time.
-
That sounds good too.
-
Yours was my second pick, though I'd say air instead of oxygen. You can "see" oxygen. Of course I guess you can technically see air too. That's why I went with time.
-
It's a hole. You can see it and it makes it lighter. Air actually adds weight.
-
Oh I was thinking the 3d. My fault.
-
That sounds correct for the 3rd.