[单选题]
When an object is 40 m in front of a con) mvue- ywc*1sej:x: xh9g)zq verging len3px;l r7y0qtm s the inverted image is half the size of the object. What is t07lx tr q;3myphe focal length of this lens?
A. $13 \; cm$
B. $20 \; cm$
C. $40 \; cm$
D. $53 \; cm$
