A stone is thrown vertically downward from the edge of a cliff with an initial speed of 10 m/s. Just before hitting the ground, it has a final speed of 30 m/s. If instead, the stone were thrown horizontally outwards from the top of the cliff with the same initial speed as before, what final speed would it have immediately before hitting the ground?
(a) 10 m/s
(b) 20 m/s
(c) 30 m/s
(d) 40 m/s