[单选题]
A person is designing a 10X telescope.u3 u2l.u6gj++ b .xevp4bpszb If the telescope is +pdhz)*yaqd /limited to a length of 20 cm, whath*)/+qa ypz dd is the approximate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
