[单选题]
A person is designin5kgwv0,posnfb l l 296g a 10X telescope. If thevyovf6-/8cv buo ;m/gz 9.ce b telescope is limited to a length of 20 cm, what is the approximate foc/ gv-v/ 6efmu.o b9;cvz8obcyal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
