Moon and Archie played chess competitively. Both of them had same levels of skill. They agreed to play seven games, and the one that win four games first would be the ultimate winner. Now, they have already played three games, and Moon won two games while Archie won one game. What is the probability that Mon be the ultimately winner? 
Overseas Competition->Knowledge Point->Counting Modules->Statistics and Probability
$$\rm B$$ 