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4. What may happen to a solid when it is continuously exposed to heat?
a. it may become heavier ​​​ c. it may expand
b. it may decrease in volume​​ d. it may condense to liquid

9. What affects the speed of sounds as it travels?
a. person receiving it​ ​​c. the origin of the sound
b. the nature of material ​​d. loudness of the sound