'are keys' or 'are a key'?

Hello, dear coaches!!!
Could anyone tell me how is it right to say:

  1. Persistence and determination are keys to success; or 2) persistence and determination are a key to success?
    Thank you very much!

Try this (someone else already has, here: Questions! What’s the difference between I have difficulty in finding the place ):

ersistence and determination are key to success.