[单选题]
When an object is placed 60 c0m/ i2-u(fe mrxdx (qsm from a converxui -p ). -/rzn2cqenlging lens, it forms a real image. Whexrp /qnz--.elu)icn2 n the object is moved to 40 cm from the lens, the image moves 10 cm farther from the lens. What is the focal length of the lens?
A. $20 \; cm$
B. $30 \; cm$
C. $40 \; cm$
D. $42 \; cm$
