The intensity at a secondary maximum of a single-slit diffraction pattern is less than the intensity at the central maximum chiefly because:
A. some Huygens wavelets sum to zero at the secondary maximum but not at the central maximum
B. the secondary maximum is further from the slits than the central maximum and intensity decreases as the square of the distance
C. the Huygens construction is not valid for a secondary maximum
D. the amplitude of every Huygens wavelet is smaller when it travels to a secondary maximum than when it travels to the central maximum
E. none of the above
