[单选题]
A person is designing a 10X telescope. Ifxy :hr3u/oh9 kobx( 0o the telescope is limited to a i6t mrtejg028 xp7j(.cg/p g qlength of t8tgx 0.6 cp7i 2g/meqgpj (jr20 cm, what is the approximate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
