[单选题]
An object is placed at a distance of 40 cm fromgvftc8l(8. y* r4 yzyf a thin lens. yq (xbtk8 m de:6q2nn, If a virtual image forms at a distance of 50 cm from the lens, on the ( n n:,q6xb2tmye8dkq same side as the object, what is the focal length of the lens?
A. $45 \; cm$
B. $75 \; cm$
C. $90 \; cm$
D. $200 \; cm$
