A might be it not to sure sorry
They have to have a higher density than water to have the ability to stay on the sea floor. Otherwise they would just float to the surface of the water
1. Well developped offspring, and portable baby food despencing. Mamalian young are generally quite well developed when they are born, and the parent does not need to find any specific types of food for the new born since the mother already provides te perfect nutrition.
2. Internal heating allows mammals to remain quick and active in cold climates.
3. Fur is really effective and cheap to produce.
Answer:
DNA that contains genes from more than one organism is called "Recombinant DNA".
