Assume there's an answer, the probability of each answer to be the correct answer:
a) 25% -> 50%
b) 50% -> 25%
c) 60% - > 25%
d) 25% -> 50%
Two possible answers: 50% and 25%,so 50% is the probability.
Oh wait, if 50% is the probability then b) is the only answer so the probability is 25%.But then again a) and b) are 25%, we go back to 50%. It's an infinite loop。