[单选题]
When an object is placed 60 cm from a converging lens, it forms e:ly*+tssj7 lh1k s 7la real imagml,q ,/kt1reh e. When the object is moved to 40 cm from the lens, the image moves 10 cm farther,e m,q/rhlt 1k from the lens. What is the focal length of the lens?
A. $20 \; cm$
B. $30 \; cm$
C. $40 \; cm$
D. $42 \; cm$
