B for me.
You work in an office with a **** of a guy who for the last 3/4 years was above you and lorded it over you acting like a fool. Now, he got a demotion and is at the same grade as you after you got a promotion. He has been a bit dodgy at work and you know he has done something you could tell the boss about. If you told the boss, the other chap would be sacked and you would get a promotion to management level - but this chaps life would fall apart and it would effect his wife, kids etc. Or you can work hard and try to get the promotion under your own steam, knowing that this guy will get demoted again at some point and probably learn his lesson this time.