[单选题]
A person is designing5 rujgtt1 :a sc0rn2 p79/4nuqhd8l jw a x0vod82tbu8 q 10X telescope. If the telescope is limited to a length of 20 cm, what is the approxidq0 o8t uv2bx8mate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
