Two Slits of Width a and Separation D Are Illuminated $\lambda$

Multiple Choice

Two slits of width a and separation d are illuminated by a beam of light of wavelength $\lambda$ .The separation of the interference fringes on a screen a distance D away is:

A) " $\lambda$ a/D"
B) " $\lambda$ d/D"
C) " $\lambda$ D/d"
D) "dD/ $\lambda$ "
E) " $\lambda$ D/a"