power 4dThe latest in indoor personalized training, the Sufferfest takes you into the 4th Dimension. Their new Four-Dimensional Power training takes an individually optimized6. A convex lens of power +6 D is placed in contact with a concave lens of power –4D. What will be the nature and focal length of this combination ? (a) convex, 100 cm (b) convex, 50 cm