Let \( f:\mathbb{R}-\{2,6\}\to\mathbb{R} \) be real valued function defined as \( f(x)=\frac{x^2+2x+1}{x^2-8x+12} \). Then range of \( f \) is
\( (-\infty,-\tfrac{21}{4}] \cup [0,\infty) \)
\( (-\infty,-\tfrac{21}{4}] \cup [\tfrac{21}{4},\infty) \)
\( (-\infty,-\tfrac{21}{4}] \cup [1,\infty) \)
\( (-\infty,-\tfrac{21}{4}] \cup (0,\infty) \)