Answer:
a) When interviewing 20 boys playing football at lunch, it is considered a random sample because you are interviewing with a large group of boys.
b) Even though I think is biased, I also think its fair, because of the fact that you are not really sure that their favorite sport is football. It doesn't say like "Oh, they must love playing football and think that's their favorite sport". So it is fair to ask this question, because maybe, that is not their favorite sport.
