The light rays are coming from infinity and are incident on lens A parallel to the axis. It will form point image at F i.e. on focal point. This F will act as source of light for lens B after refraction B, the emergent ray will parallel to axis of lens. Similarly, after refraction from C, they will converge at F. This F will act as a source. After refraction from D, the light rays will emerged parallel to the axes of lens.