Figure P12.16 shows a coiled compression spring that has been loaded against a support by means of a bolt and nut. After the nut has been tightened to the position

shown, an external force F is applied to the bolt as indicated, and the deflection of the spring is measured as F is increased. A plot of the resulting force-deflection curve is shown. Briefly but clearly state the reasons the curve changes at points A, B, and C.