Which description best represents the variation of kinetic energy K with instantaneous displacement x (measured from the mean position) for a block moving in simple harmonic motion attached to an ideal spring?
a) a parabola opening upward with its vertex at x=0
b) a horizontal straight line at K=21kA2
c) a parabola opening downward with its vertex at x=0
d) a straight line passing through the origin with positive slope