[单选题]
A double convex lens made of glass (n = 1.50) has a rad2cwpr3u7p rk+* p,ez qius of 40 cm on t ak 7 wqht1s7itd5y1*khe front side and 30 cm on the7hasyitkd qk t5 71*1w back side. What is the focal length of the lens?
A. $11 \; cm$
B. $34 \; cm$
C. $80 \; cm$
D. $240 \; cm$
