Which of the following is the unbounded set ?
$ X = \{ x\mid x = \frac{1}{n},n \in \mathbb{N} \} $
$ Y = \{ x\mid x = \frac{1}{2^n},n \in \mathbb{N} \} $
$ Z = \{ x\mid x = 2^n,n \in \mathbb{N} \} $
$ W = \{ x\mid x \in \mathbb{N}, x \lt 4532 \} $
After reading the definition from Wikipedia , I can only convince myself that it can't be W but am not getting which is correct from the other three ?