Different kinds of animals have appeared and disappeared throughout Earth’s history. Some animals go extinct because the climate (1)_______ they live changes. The climate may become wetter or drier. It may become warmer or cooler. If the animals cannot change, or adapt, to the new climate, they die. Some animals go extinct because they cannot (2) _______ with other animals for food. Some animals go extinct because they are killed by enemies. New kinds of animals are always evolving. Evolving means that the animals are changing (3) _______ from generation to generation. Small differences between parents, children, and grandchildren slowly add up over many, many generations. Eventually, a different kind of animal evolves. Sometimes many of the animals on Earth go extinct at the (4) _______ time. Scientists call this a mass extinction. Scientists think there (5) _______ at least five mass extinctions in Earth’s history. The last mass extinction happened about 65 million years ago. This mass extinction killed off the dinosaurs.

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(4)