A body, moving in a straight line, with an initial velocity u and a constant acceleration a, covers a distance of 40 m in the 4th second and a distance of 60 m in the 6th second. The values of u and a respectively are (a) 10 ms-1 5 ms-2 (b) 10 m s-1, 10 m s-2 (c) 5 ms-15m: 8-2 (d) 5 m s-1, 10 m s-2