Coherent light of a single frequency passes through a double slit, with slit separation d, to produce a pattern of maxima and minima on a screen a distance D from the slits. What would cause the separation between adjacent minima on the screen to decrease?

(a) Decrease the frequency of the incident light
(b) Increase of the screen distance D
(c) Decrease the separation d between the slits
(d) Increase the index of refraction of the medium in which the setup is immersed