Aşağıdakilerden hangisi boş kümedir?
A) $\emptyset$
B) $\left\{ x : x^2 = 0, x \in \mathbb{R} \right\}$
C) $\left\{ x : x < 5, x > 7, x \in \mathbb{Z} \right\}$
D) $\left\{ x : x + 2 = 5, x \in \mathbb{N} \right\}$
E) $\left\{ x : 2x = 4, x \in \mathbb{Z} \right\}$
