[单选题]
A double convex lens made o y0zjrqnj;7*v5ao e :nf glass (n = 1.50) hascf ys2 :5-cjgs a radius of 40 cm on the front side and 30 cm on the back side. What is the focal length5 f scj-gscy:2 of the lens?
A. $11 \; cm$
B. $34 \; cm$
C. $80 \; cm$
D. $240 \; cm$
