A spring of stiffness k is compressed by a length 
and a ball is placed on the top of it. When the spring is released the ball is seen to rise through a height h. When two such springs are kept one over the other, and the combination is compressed by same length $\bigtriangleup$ , what is the height reached by the balt kept on the combined springs when released?
(a) h/2 (b) h (c)$\sqrt{2}h$ 
(d) 2h