[单选题]
An object is placed at auno ifgf1du;0cf-l2ur h*5r9 distance of 40 cm from a thin lens. If a virtual im*; mewgt1)*m .qmmfu/v nl;pyage forms at a distance of 50 cm from the lens, on the same side as the object, wha ;lmmftm/ ngw.** vuqpe) ;1myt is the focal length of the lens?
A. $45 \; cm$
B. $75 \; cm$
C. $90 \; cm$
D. $200 \; cm$
