If the normal at meets the major axis of an ellipse at Q and S and F1 is the focus lying on the positive side of the x-axis, and F2 is the focus lying on the negative side of the x-axis, then:
a) F1Q + F2S is constant
b) F1Q - F2S is constant
c) F1Q × F2S is constant
d) F1Q / F2S is constant
