The system of equations $\alpha x+y+z=\alpha-1$, $x+\alpha y+z=\alpha-1$, $x+y+\alpha z=\alpha-1$ has infinite solutions if $\alpha$ is

Question

$(a)\;-2\qquad(b)\;either \;-2\;or\;1\qquad(c)\;not\;-2\qquad(d)\;1$