A person standing at the foot of a tower walks a distance 3a away from the tower and observes that the angle of elevation of the top of the tower is α. He then walks a distance 4a perpendicular to the previous direction and observes the angle of elevation to be β . The height of the tower is
(A) 3a tan β
(B) 5a tan β
(C) 4a tan β
(D) 7a tan β
