[单选题]
A person is designing a 10Xs170m46sz 3i tftl 5nrvkd .ml telescope. If the telescope is lq;0/.gb bgzlxx :-au jimited to a length of 20 cm, what is the approbgz;l-: j0 /.abguqx xximate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
