What Enables a Variable-Rate,multi-Leaf Spring to Become Progressively Stiffer as Weight

What enables a variable-rate,multi-leaf spring to become progressively stiffer as weight is added to a frame?

A) cam brackets make contact close to the center of the spring as it deflects
B) the ends of lower leaves in the spring pack come into contact with the upper leaves
C) both A and B
D) neither A nor B