[单选题]
A double convex lens made of glass (n = 1.50) has a radius o6 s1,d7wcv9lfue y m3e,us;m p5kkqz t gd: 2we.4of8nf 40 cm on the front side and 30 cm on the,kf qt4wg25pmo:sned.k8; z u back side. What is the focal length of the lens?
A. $11 \; cm$
B. $34 \; cm$
C. $80 \; cm$
D. $240 \; cm$