A lot of people are interested in sea animals, especially the ones that are beautiful and smart. Although some are wild, and can be deadly, each is also unique in its own way. Whales, sharks and dolphins are the main subjects of sea-animal research, with dolphins standing out from whales and sharks for their friendly behavior with humans. No doubt the desire to learn some interesting things about dolphins has led you to this article, and you too are eager to know more about these sea creatures.      It is sometimes believed that Dolphins have been evolved from land mammals  . Among sea mammals, dolphins are the most advanced in intelligence. You may be surprised to learn that they are close  s of camels and cows. The debate on the origin of the dolphin is an unending one, however. Some believe that they evolved from land-based hoofed   mammals, others believe that they evolved from other different kinds of mammals many millions of years ago. There are about 32 types of dolphins that live in salt water, each unique in its color1 and body shape. Five other types live in fresh water.       A grown-up dolphin eats nearly 20 to 22 kilos of fish every day. The eating habits of dolphins depend on where they live, and on the season. Seasons in which fish are abundant are like party time for dolphins. Dolphins decide how much they are going to eat based on the fat content of the fish available. They've a good sense of understanding about their hunger.      The social skills and intelligence of dolphins make them very attractive for zoos and aquariums, where they are often trained to show off their diving skills. Dolphins also communicate with each other by special sounds. Scientists like to research dolphins mainly because   _  .
they are friendly to humans they have good diving skills they have unique bodies they are beautiful and clever
Question: A lot of people are interested in sea animals, especially the ones that are beautiful and smart. Although some are wild, and can be deadly, each is also unique in its own way. Whales, sharks and dolphins are the main subjects of sea-animal research, with dolphins standing out from whales and sharks for their friendly behavior with humans. No doubt the desire to learn some interesting things about dolphins has led you to this article, and you too are eager to know more about these sea creatures.      It is sometimes believed that Dolphins have been evolved from land mammals  . Among sea mammals, dolphins are the most advanced in intelligence. You may be surprised to learn that they are close  s of camels and cows. The debate on the origin of the dolphin is an unending one, however. Some believe that they evolved from land-based hoofed   mammals, others believe that they evolved from other different kinds of mammals many millions of years ago. There are about 32 types of dolphins that live in salt water, each unique in its color1 and body shape. Five other types live in fresh water.       A grown-up dolphin eats nearly 20 to 22 kilos of fish every day. The eating habits of dolphins depend on where they live, and on the season. Seasons in which fish are abundant are like party time for dolphins. Dolphins decide how much they are going to eat based on the fat content of the fish available. They've a good sense of understanding about their hunger.      The social skills and intelligence of dolphins make them very attractive for zoos and aquariums, where they are often trained to show off their diving skills. Dolphins also communicate with each other by special sounds. Scientists like to research dolphins mainly because   _  .
A. they are friendly to humans
B. they have good diving skills
C. they have unique bodies
D. they are beautiful and clever
Answer: