[单选题]
A person is designing a 10X telescope. If the telescope is limite v,e6 a1enaeg: /yo7br;m0z 60vuqlqud to 1y-.yojl+f qstr4w u( a length of 20 cm, what is the approximate focal length of the ow uo-y4 lfsqry(+j.1tbjective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
