[单选题]
An object is placed at )h/pvu ;9 kjng1lcdd)zjt-h;a distance o(5*y imrk (p9wr4ro mff 40 cm from a thin lens. If a virtual image forms at a distance of 50 cm from the lens, on the same side as the o4rmm* r p (rwfo(k59iybject, what is the focal length of the lens?
