[单选题]
A double convex (conve9aa. 7ckhc q1ox-rt jws0ob( s.-2g6o ebconvex) thin lens has radii of curvature 46 cm, and is mo- (e2gbwrj0sbo.t s6ade of glass of index of refraction n = 1.60. What is the focal length?
A. infinite
B. $38 \; cm$
C. $30 \; cm$
D. $18 \; cm$
