if every continuous characteristic function is constant then ,M is connected

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prove for a metric space $M$, if every continuous characteristic function is constant then $M$ is connected.
I actually know how to prove the other direction, but I do not know how to work on this direction.

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Suppose $M$ is not connected. Let $A$ be a connected component of $M$ and note that $A$ is open in $M$. Then $1_M$ is continuous but not constant.