What is the **least number of unit squares that you need to remove** from this 5 by 3 checker board in order to make it *impossible* for anyone to put an "X" in 3 remaining squares to make a connected vertical, horizontal, or diagonal set of 3 (a set that looks like a "win" in a standard Tic Tac Toe game)?

Hint: Try the problem on a smaller board first.

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