[单选题]
A person is designing r 2ztc9fkg(4 t 6nla4la ;;/podh2rtyi nu2 4byb h-tayyf07:t 10X telescope. If the telescope is limited to a length of 20 cm, whabto0i7 n d2 yu yf4 trthyb;2;-ap:h/yt is the approximate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
