A bungee cord exerts a nonlinear elastic force Fx​=k1​x+k2​x3 where x is the change in the length of the cord from its unstretched length.
a) k1​x+k2​x3
b) k1​x−k2​x3
c) k1​x×k2​x3
d) k1​ˣ⁄ₖ2​x3