[单选题]
A person is designing a 10X telescope. If the telescope is lt qfflxom9 xf.5 2e3p/imited toh8c+kzuub 7d) a length of 20 cm, what is th)du7k+czbhu 8e approximate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
