I'm taking a test and have to guess the right answer to the question after the article. Article: Many birds migrate. This means that every year they fly somewhere warm in the winter and then return to the place they came from in the spring. One mystery is how birds know how to travel such long distances without getting lost. Scientists have wondered for years whether they are born with the knowledge of where to go or whether they learn it after they are born. Now, researchers have found evidence that shows that one type of bird, at least, learns how to migrate. The bird is the whooping crane , a tall, white bird that lives in the United States. In the 1940s, the number of these birds decreased to fewer than 25 individuals, and many people feared that they would die out. Thanks to efforts to save them, they have now increased to about 600. However, the birds are not very good at taking care of their young. Because of this, eggs are brought to a site in Maryland. There, scientists from the Whooping Crane Eastern Partnership raise the baby birds. Later, the young birds are taken from Maryland to Wisconsin, where the scientists teach them how to fly. From there, they fly about 2,200 kilometers south to Florida to spend the winter. The first time, scientists lead the way in a small airplane. From the second year, though, the birds travel by themselves. For 14 years, the scientists have kept a careful record of every flight the birds have taken between Wisconsin and Florida. They have also recorded which birds fly together, the relationships between the birds, and the gender of each bird. When scientists analyzed the data, they were surprised by the results. These showed that each bird took a very different route. However, as they got older, they flew more and more directly between Wisconsin and Florida. This continued until they were five years old. This indicates that the birds gradually learned the best route to take. Moreover, when an older bird was in a group, the whole group flew more directly. This suggests that the older birds taught the younger ones a quicker route. Scientists say that this evidence... Question: Why do scientists raise whooping cranes? Options: A: They want to observe how the mothers take care of their young. B: They are worried that the birds will fall down many times. C: Whooping cranes' eggs need to be watched constantly. D: Whooping cranes are bad at raising their own young.
A:
D