A highly poisonous animal X which inhabits crop fields is commonly known as friend of the farmer. It eats up pests like Y and saves the crops from damage. The animal X is killed in large numbers to obtain its Z which sells in the market at a high price . What are X, Y and Z ?

There were 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 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.