[单选题]
A double convex (convexzo5x5/yg i.qk-convex) thin lens has radpg/*cytf3z) xvi)u9qo2/k isii of curvature 46 cm, and is made of glas uf/y3c/9*pzs2 )vit)qgoixks of index of refraction n = 1.60. What is the focal length?
A. infinite
B. $38 \; cm$
C. $30 \; cm$
D. $18 \; cm$
