[单选题]
An object is placed at a distafx625pwnur8uq, j4yt nce of 40 cm xs * (gwvbzi rn3q0+q7from a thin lens. If a virtual image forms at a distance of 50 cm from the lens, o 7r+0n*s3w(xgzqv iqbn the same side as the object, what is the focal length of the lens?
A. $45 \; cm$
B. $75 \; cm$
C. $90 \; cm$
D. $200 \; cm$
