[单选题]
When an object is 40 m in y1tk7 a; ijtnn9fj2e-front of a converging lens -ms* .d:,;pqhynvlub the inverted image is half the sizey*vhpn,s u;dq: . b-ml of the object. What is the focal length of this lens?
A. $13 \; cm$
B. $20 \; cm$
C. $40 \; cm$
D. $53 \; cm$
