[单选题]
A person is designing a 10X telescope.u7k)d1 ,vb/hqkdyi2 p If81/.;n kh)gnfjnw bhb the telescope is limited to a length of 20 cm, what is the approximate focal length of the objectin)gkh hnb/1b;fwn. j 8ve?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
