Suppose you are participating in a game program, and you are asked to choose one of the three gates: there is a car behind one of the gates and the others~ are goats. You choose a gate, as gate $$1$$, and then the host who knows what is behind the door opens gate $$3$$ with a goat behind it. He then asks you, "Do you want to choose gate 2?" At this time,which gate should you choose? 
Overseas Competition->Knowledge Point->Counting Modules->Statistics and Probability