Mark the letter A, B, C, or D on your answer sheet to indicate the best arrangement of utterances or sentences to make a meaningful exchange or text in each of the following questions from 13 to 17.

a. Since life began on Earth, scientists think that at least five billion kinds of plants and animals have gone extinct.
b. Some species died out slowly because conditions changed. Others were killed in a huge natural disaster that happened about 66 million years ago.
c. Many species are dying out these days, so some scientists think another mass extinction is happening now. They see a relationship between human activity and these deaths.
d. For example, they think the problem is happening because humans are causing climate change.
e. If a scientist found a way to stop climate change, he or she would be a hero to people who care about the planet.
(Adapted from Explore New World)