The answer is B every single time. The runner on 3rd base has to go. If they don't throw- run scored. If they do, the runner gets in a run-down. His job is to stay in it long enough for runners to move up to 2nd and 3rd. Runner on 1st and hitter should have been taught what to do previously in practice. First base coach points to 2nd immediately to runner to alert him of a run-down.