Two ideal slits S1 and S2 are at a distance d apart, and illuminated by light of wavelength l passing through an ideal source slit S placed on the line through S2 as shown. The distance between the planes of slits and the source slit is D. A screen is held at a distance D from the plane of the slits. The minimum value of d for which there is darkness at O is

• A.            \[\sqrt{\frac{3\lambda D}{2}}\]
• B.            \[\sqrt{\lambda D}\]
• C.            \[\sqrt{\frac{\lambda D}{2}}\]
• D.            \[\sqrt{3\lambda D}\]