In a △ABC with fixed base BC, the vertex A moves such that cosB + cosC = 4sin2A/2. If a, b and c denote the lengths of the sides of the triangle opposite to the angles A, B and C respectively, then
(a) b + c = 4a
(b) b + c = 2a
(c) locus of point A is an ellipse
(d) locus of point A is a pair of straight line
