How are weeds removed from the crop fields ?

What will happen if the weeds are not removed from the crop fields?

Which of the following tools would a farmer use to remove weeds from the field?

why the weeds are removed ?

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.