[单选题]
A biconvex lens is formed by using a piece om cz0nx83j ;hjf plastic (n = 1.70). The z9u;iaeyd jlf7 ma5/8 ; ytzq2radius of the front surface is 20 cy7y9 2m8j5ae;lq/zz uf i;ad tm and the radius of the back surface is 30 cm. What is the focal length of the lens?
A. $17 \; cm$
B. $86 \; cm$
C. $-86 \; cm$
D. $-17 \; cm$
