Aşağıdaki bağıntılardan hangisi bir fonksiyon belirtir?
A) $f: \mathbb{R} \to \mathbb{R}, f(x) = \frac{1}{x-2}$
B) $f: \mathbb{N} \to \mathbb{Z}, f(x) = \sqrt{x-1}$
C) $f: \mathbb{Z} \to \mathbb{Z}, f(x) = \frac{x+1}{2}$
D) $f: \mathbb{R} \to \mathbb{R}, f(x) = x^2+1$
E) $f: \mathbb{R} \to \mathbb{R}, x^2+y^2=4$
