Free-living flatworms have developed a nervous system which helps them to sense the food. They have a head at one end of the body and each side of the head has sensory cells. These cells help them to sense chemicals and other movements around them.
While Parasitic flatworms don't have any nervous or digestive system as they live on the food digested by the host. They have special hooks attached to their head which help them to attach a host. They have developed some covering called 'teguments' which protect them from digestive liquids.
The answer is B. artificial selection. Artificial selection is . a process in the breeding of animals and in the cultivation of plants by which the breeder chooses to perpetuate only forms that have certain desirable inheritable characteristics (such as, in this case, cows that produce the most milk).
