As to your question on bitcoin mining, I think you meant B to be the correct answer, but more correctly it's not guessing a "long" number. It's guessing a low number below a given threshold known as the difficulty standard (which is adjusted every 2 weeks). The nonce is the name of that low number. Essentially, bitcoin mining is playing a game of limbo where the bar is so close to the floor that only virus particles can get through.