Good Riddles

Question: I can be one, I can be two. I can be Open, Closed, Empty or Full. I can be Bare, but never a Bear. What AM I??

Question: What is AD before BC, exams before tests and last year is after last month?

Question: what does not move outside (the surface)but moves inside ?

Question: what have holes but you will see it on the street ?

Question: Only one color, but not one size, Stuck at the bottom, yet easily flies, Present in sun, but not in rain, Doing no harm, and feeling no pain. What am I?