Explain how the speed of light changes between a gas liquid and solid

As a rule sound travels slowest through gases, faster through liquids, and fastest through solids. The speed of light as it travels through air and space is much faster than that of sound; it travels at 300 million meters per second or 273,400 miles per hour.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.

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