If someone has a heavy weight on his or her chest, explain why after a short time it becomes difficult to breathe, even though the person’s mouth and nose are not affected

Breathing problems are when you feel you can’t get enough air, your chest feels very tight, you are breathless or you feel like you’re being suffocated.

You might feel short of breath if you are obese or if you have just done some strenuous exercise. It can also happen in extreme temperatures or if you are at high altitude.

If you have breathing problems for any other reason, it is probably the sign of a medical problem.

Hope it helps :)

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a.) a destruction fo cells by viral reproduction

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.