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