[" 15.If the line "x cos theta+y sin theta=2" is the equation "],[" transverse common tangent to the circles "x^(2)+y+0" ,then the value "],[" and "x^(2)+y^(2)-6sqrt(3)x-6y+20=0" is "],[[" is "," a."5 pi/6," b."2 pi/3," c."pi/3]]