A bungee jumper of mass m jumps off a bridge. Assume that the bungee cord behaves like am ideal spring of spring constant k. When the jumper is at the lowest point in the motion that follows, the extension of the bungee cord is x.
After reaching the lowest point for the first time, the jumper rebounds straight upwards.
What is the magnitude of her velocity at the instant that the cord is at its natural length?

a) √(k/m) x
b)√[(kx²/m) - 2gx]
c) √(2gx)
d) √(-2gx)
e) None of the above