Let $$f: R - \{2, 6\} \to R$$ be real valued function defined as $$f(x) = \dfrac{x^2+2x+1}{x^2-8x+12}$$. Then range of $$f$$ is

A

$$\left(-\infty, -\dfrac{21}{4}\right] \cup \left[\dfrac{21}{4}, \infty\right)$$

B

$$\left(-\infty, -\dfrac{21}{4}\right] \cup [0, \infty)$$

C

$$\left(-\infty, -\dfrac{21}{4}\right) \cup (0, \infty)$$

D

$$\left(-\infty, -\dfrac{21}{4}\right] \cup [1, \infty)$$