There were a lot of organisms A in the crop fields in an area. These organisms used to feed on organisms B also present in the crop fields but which damaged the standing crops. One day some people arrived in the fields and killed all the organisms A so that they could remove their C for making fancy items which were in great demand. Due to the large-scale killing of A, the population of B increased too much damaging all the crops in the area and causing a famine-like situation. Apart from A, there is another organism D which can also kill and eat B. Name A, B, C and D.
Organism A is a snake as it feeds on rats and it is being killed to make fancy items.
Organism B is a rat.
Organism C is the skin of a snake that is used in making fancy items.
Organism D is a cat as it also feeds on rats (B).