The Figure Shows a Doublet Lens in Which the Lens

The figure shows a doublet lens in which the lens with positive focal length and the lens with negative focal length are both manufactured from flint glass and have the same radii of curvatures. The focal length of the concave lens is -50.8 cm, and the focal length of the convex lens is 50.8 cm. The combined focal length of the doublet is

A) -25.4 cm.
B) 25.4 cm.
C) -102 cm.
D) 102 cm.
E) infinite.