[单选题]
When an object is placed 60 cm vmj. j /pi*a20suf8adfrom a convbg0wybbf;eoi42+kk /erging lens, it forms a real image. When the object is moved to 40 cm from the lens, the image moves 10 cm farther from the len bb/i;yg2 be ko0kwf+4s. What is the focal length of the lens?
A. $20 \; cm$
B. $30 \; cm$
C. $40 \; cm$
D. $42 \; cm$
