Correct Answer - B
u=40 m/s , `g=10 m//s^(2)`
Let t be time taken by the first ball to reach the highest point.
`V=-u-g t " " O=40-10 t " " t= 4s`
After reaching the first ball at the highest point now, both the balls will collide after 1 sec as both the balls therefore, the height of collision point =height gained by the second ball in 3 sec