If the three distinct lines $\;x+2ay+a=0\;,x+3by+b=0\;$ and $\;x+4ay+a=0\;$ are concurrent , then the point (a,b) lines on a :

Question

$(a)\;circle\qquad(b)\;hyperbola\qquad(c)\;straight\;line\qquad(d)\;parabola$