[单选题]
A person is designing a(uy(c7cn hvv( the .+m 101s)*ru84 4alpcy(1*wych d h6kbhm tkX telescope. If the telescope is limited to a length of 20 cm, what is the approximate focal lengthrhalswc 1)uyht** (k m64d1 pkc84bhy of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
