[单选题]
A person is designing a 10X tvkvuv*n arp+:c :w3 .nzt )8ny+o2mow elescope. If the telescope is limdn* nbws1qk tv) ;7s34;hspaaited to a length of 20 cm, what is the approximate focal lengths3*7san ;; aphs 1 )4tqnvbwkd of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
Post time 2024-1-26 22:03:25