In a Young's double-silt experiment the source slit S and the two slits A and B are horizontal with slit A above slit B. The fringe are observed on a vertical screen. The optical path length from S to B is increased slightly by introducing a tranparent slab. As a result the fringe pattern on the screen moves A. vertically downwards B. vertically upwards C. horizontally to the left D. horizontally to the right
