A Young’s double – slit set up for interference is shifted from air to within water. Then the (A) Fringe pattern disappear (B) fringe width decreases (C) fringe width increases (D) fringe width remains unchanged

In double –slit experiment the distance of the second dark fringe from the central line is 3 mm. The distance of the fourth bright fringe from the central line is (A) 6 mm (B) 8mm (C) 12 mm (D) 16 mm