DS Goes Options Trading

[Dismal Science]

Here is an interesting question I have - how do the exchanges determine which strikes to offer for options? For example - yesterday the lowest weekly put option strike on AAPL expiring 4/27 was $550. Today, it looks like additional strikes are offered - going all the way down to $520.

I might end up just selling put options that expire in May with a strike about $100+ dollars away from where AAPL is currently trading. The weekly $520 strike put is tempting but risky.