[单选题]
A person is designing a 10X telescope. If the 6s yj s06c- u/ -n+umrws8klyntelescope is limited to a len3t., f.fh:vtxb e-jabgth of 20 .b-hfbx.fta3:tv e,jcm, what is the approximate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
