This problem is very complicated because human factors have changed his uncertainty.

It is found that no matter which one is chosen, the correct rate is only 1/3. But because the host knows the answer. He helped you get rid of a wrong answer. Then the remaining correct rate of your choice is 1/2. Then the key to the problem is that the host knows the answer and thinks it changes the probability.

Don't doubt me, the probability will definitely change. I didn't believe it at first, but then I did the experiment myself.

I drew a circle on one of the three pieces of paper for my classmates to demonstrate with me. Results I did 100 experiments and found that the accuracy of not telling the answer was only 1/3. After she helped me eliminate it, the correct rate was 1/2.