[单选题]
A person is designing a 10X cyw ky f(n7+o*6uf:vwtelescope. If a q)yuficz0hbx 1+.g) the telescope is limited to a length of 20 cm, w0 .f) gqabzxi)c yhu+1hat is the approximate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
