Questions 19 to 22 are based on the conversation you have just heard.

　　19. [A]Two different types of bones in the human body.

　　[B]How bones help the body move.

　　[C]How bones continuously repair themselves.

　　[D]The chemical composition of human bones.

　　20. [A]They defend the bone against viruses.

　　[B]They prevent oxygen from entering the bone.

　　[C]They break down bone tissue.

　　[D]They connect the bone to muscle tissue.

　　21. [A]They have difficulty identifying these cells.

　　[B]They aren't sure how these cells work.

　　[C]They've learned how to reproduce these cells.

　　[D]They've found similar cells in other species.

　　22. [A]To learn how to prevent a bone disease.

　　[B]To understand differences between bone tissue and other tissue.

　　[C]To find out how specialized bone cells have evolved.

　　[D]To create artificial bone tissue

　　Questions 23 to 25 are based on the conversation you have just heard.

　　23. [A]A new fuel for buses. [B]The causes of air pollution.

　　[C]A way to improve fuel efficiency in buses. [D]Careers in environmental engineering.

　　24. [A]Her car is being repaired. [B]She wants to help reduce pollution.

　　[C]Parking is difficult in the city. [D]The cost of fuel has increased.

　　25. [A]A fuel that burns cleanly.

　　[B]An oil additive that helps cool engines.

　　[C]A material from which filters are made.

　　[D]An insulating material sprayed on engine parts.