[单选题]
A person is designing a 10X telescoprkkc -a*5djq 3e. If the telescope is limited to a lengthom hxb9)m ffd6:y):tr of 20 cm, what is the appro:fbd x9moh: )fty 6)rmximate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
