[单选题]
A person is designing a 1sma9z2 4gsq ay0dt 27qw*2s sa0Xmv8np 0qrb +r8 telescope. If the telescope is limited to a length of 20 cm, what is the approximbrvp+08 q8nr mate focal length of the objective?
A. $16 \; cm$
B. $17 \; cm$
C. $18 \; cm$
D. $19 \; cm$
