You combine lens. A and lens B by placing them in contact so that their axes coincide, forming what is called a compound lens. The focal length of lens A is 10 cm; the focal length of lens B is unknown. When a 5−cm tall real object is placed at 10 cm from the compound lens, an upright image with a height of 8 cm is formed. What is the focal length of lens B ? −16 cm −32 cm 24 cm 8 cm −8 cm 12 cm