Aşağıdakilerden hangisi bir fonksiyon belirtmez?
A) $f: \mathbb{R} \to \mathbb{R}$, $f(x) = x+3$
B) $f: \mathbb{N} \to \mathbb{N}$, $f(x) = x-1$
C) $f: \mathbb{Z} \to \mathbb{Z}$, $f(x) = 2x$
D) $f: \mathbb{R} \to \mathbb{R}$, $f(x) = x^2$
E) $f: \mathbb{R}^+ \to \mathbb{R}$, $f(x) = \sqrt{x}$
