article: Dolphins are one of the cleverest animals in the world. We may hear of many stories about how dolphins save people out of water.
  Dolphins often eat fish and some other sea animals. A dolphin with a weight of 200kg to 250kg will eat 10kg to 25kg of fish every day. They like to swim with other dolphins and usually hunt in groups.
  Dolphins have very good hearing. They use it to find things under the water. Many scientists also believe dolphins can even talk to each other through a special whistle . People from different areas don't speak the same language. Just like people, dolphins' whistles have many kinds. One day we may understand dolphins' whistles, and then they will be able to tell us about themselves.
  Dolphins live in the sea, but they are not a kind of fish and they don't lay eggs. They have babies in the same way as cows and cats do, and give their babies milk. A female dolphin has a baby every two to three years.
  Dolphins are very friendly. Sadly, there are not many kinds of dolphins left. People killed them for many years for food and other things. Now it's time to stop this and perhaps there will be more of them in the years to come.
question: Which of the following is an opinion ?
options: Dolphins eat fish and some other sea animals. Dolphins are not a kind of fish. Dolphins usually hunt in groups. People may understand dolphins' whistles some day.