A converging lens of focal length 50 cm is placed

A converging lens of focal length 50 cm is placed coaxially in contact with another lens of unknown focal length. The combination behaves like a diverging lens of focal length 50 cm, what is the power and nature of second lens ?

a) Convex, + 4D b) Concave -4D
c) Convex, + 3 D d) Concave, + 5

1/f = 1/f1 + 1/f2
-1/50 = +1/50 + 1/f2
So, 1/f2 = -1/25 (1/cm)
So, Power = -100/25 = -4D