38. ___
A. The high temperatures required for its use.
B. The high cost of materials used in its production.
C. The lack of trained environmental engineers.
D. The opposition of automobile manufacturers.
31. ___
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.
32. ___
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.
33. ___
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.
34. ___
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.
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