The range of $y=\large\frac{x^2-x+1}{x^2+x+1}$ if $x$ is real

Question

$\begin{array}{1 1}(A)\;\big[-\large\frac{1}{3},\normalsize 3\big]&(B)\;\big[\large\frac{1}{3},\normalsize 3\big]\\(C)\;\big[ 3,-\large\frac{1}{3}\big]&(D)\;[-3,3]\end{array} $