[单选题]
An object is placed at a distance of 40 kfqua8*5-np+dwp,mx cm i 0p4sjgnk42j from a thin lens. If a virtual image forms at a distance4gpsj20ji kn 4 of 50 cm from the lens, on the same side as the object, what is the focal length of the lens?
A. $45 \; cm$
B. $75 \; cm$
C. $90 \; cm$
D. $200 \; cm$
