How Is Someone Selected To Be A Top-Contributor on Yahoo Answers & Questions?

I already have read the information from the Help Section of Yahoo pertaining to Top-Contributors. The explaination is rather short & still leaves me with more questions. I am wondering about more details: Who exactly chooses you? - How many times approximately must you respond to a question regarding a particular subject before your are chosen? - Is there anything else that is required of oneself in order to be designated a Top-Contributor? Thank You to All who respond here.