When an object is placed 750px from a converging lens, the image formed is positioned 1500px from the lens. If the object is moved 125px closer to the lens, the position of the image changes by 40 cm. What is the focal length of the lens?
A、16 cm;
B、25 cm ;
C、36cm;
D、32cm;
E、20cm
发布时间：2025-07-25 18:58:38
