A man is walking towards a vertical pillar in a straight path, at a uniform speed. At a certain point A on the path, he observes that the angle of elevation of the top of the pillar is 30o. After walking for 10 min from A in the same direction, at a point B, he observes that the angle of elevation of the top of the pillar is 60o. Then, the time taken (in minutes) by him, from B to reach the pillar, is
(a) 6
(b) 10
(c) 20
(d) 5
