Long Riddles and Answers

Question: Whats a question that gets asked a lot and has many different answers to it.

Question: What are the next two letters in the following series and why? WATNTLITFS__

Question: It is always bought when you're hungry, but it doesn't get eaten. What am I?

Question: What is often picked but seldom chosen, never walks but can run when frozen?

Question: I have Eighty-eight keys but cannot open a single door?

What am I?