Friction produces --- and wear. a. force b. tear c. heat d. cold

Here are the answers to your questions:

8. Friction produces heat and wear. The correct option is $\boxed{\text{C. heat}}$

9. The hotness and coldness of an object is known as temperature. The correct option is $\boxed{\text{C. temperature}}$

10. The life process where living things increase in size is growth. The correct option is $\boxed{\text{D. growth}}$

11. The liver removes poisonous chemicals from digested food. The correct option is $\boxed{\text{C. liver}}$

12. Plants make their own food in a process called photosynthesis. The correct option is $\boxed{\text{B. photosynthesis}}$

13. The roughness or smoothness of a surface is known as texture. The correct option is $\boxed{\text{A. texture}}$

14. The instrument used to measure temperature of a body is known as Thermometer. The correct option is $\boxed{\text{C. Thermometer}}$

15. The normal body temperature of a human being is known 37°C. The correct option is $\boxed{\text{C. 37°C}}$

16. Switch is used to control the flow of current in an electric circuit. The correct option is $\boxed{\text{C. Switch}}$

17. It takes the earth 24 hours to complete one rotation cycle. The correct option is $\boxed{\text{A. 24 hours}}$

18. The symbol $\otimes$ is a bulb. The correct option is $\boxed{\text{B. bulb}}$