In a discrete metric space M any open Ball is
Correct Answer: 

C) either a singleton set or the whole space 

Description for Correct answer:
Let \(x\in M\)
Then \(B(x,r)=\{x\}\) if \(r<1\)
\(B(x,r)=M\) if \(r\ge 1\)
Therefore in a discrete metric space M any open ball is either a singleton set or the whole space.
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