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A: Who's up for a game of pool?

B: I am.

C: Yeah, why not?

A: Tim?

Tim: No, I'm good.

- Is 'no, I'm good' natural to use here?

- Does 'why not' always need a question mark after it or is this a situation where it can be omitted? What would you do?

Thanks.

Comments  
anonymous- Is 'no, I'm good' natural to use here?

Yes, in casual speech among some people, certainly among the crowd in a pool hall.

anonymousDoes 'why not' always need a question mark after it

Yes. It is a question.

anonymousor is this a situation where it can be omitted?

The only time you can and should omit the question mark after what is to all appearances a question is in polite formulas like "Would you be so kind as to …." As far as I know.