[单选题]
When an object is 40 m in front of a conv-v.n/d rqm euemfqk+5ab;4 d6ergijwi1g r)8cn x2ng lens the inverted image is half the size of the object. What is the focal length of gc)8njriw1 2xthis lens?
A. $13 \; cm$
B. $20 \; cm$
C. $40 \; cm$
D. $53 \; cm$
