Which is a singleton set?

Which is a singleton set?
  1. $\{x : x \in \mathbb{N},\ 2 < x < 4\}$
  2. $\{x : x \in \mathbb{N},\ 0 < x < 4\}$
  3. $\{x : x \in \mathbb{N},\ x < 1\}$
  4. $\{x : x$ is a prime, $x < 10\}$

Solution

$\{3\}$ — only one element.

Asked in: MH-SSC-9

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