Two lenses have powers of $(a) +2 \text{ D}$ and $(b) -4 \text{ D}$. Find the nature and focal length of each lens.

• A
  Convex,$0.5 \text{ m}$ and Concave,$-0.25 \text{ m}$
• B
  Concave,$0.5 \text{ m}$ and Convex,$-0.25 \text{ m}$
• C
  Convex,$0.25 \text{ m}$ and Concave,$-0.5 \text{ m}$
• D
  Concave,$0.25 \text{ m}$ and Convex,$-0.5 \text{ m}$