[单选题]
A biconvex lens is fod1zx yl.bo8j6 rmed by usinmf o9mt8 ;txy1y;*qh+ ir az,pg a piece of plastic (n = 1.70). The radius of the front surface is 2; 8yo txp,hifytrm *m;z9q+a10 cm 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. $-17 \; cm$
