[单选题]
A person is designing a 10X telescope. If the telescope is limited to a e,g2;311t71 luzykldoe jo hdlev1)uhvf1d/.aworq4p (x w*z v ngth of 20 cm, what is the approximate focal length of thhfr.vpaw(u xo )qwd14v/* 1 zve objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
