Troposphere should be the answer
Answer:The insurance that pays first (primary payer) pays up to the limits of its coverage. The one that pays second (secondary payer) only pays if there are costs the primary insurer didn't cover. ... If your employer insurance is the secondary payer, you may need to enroll in Medicare Part B before your insurance will pay.
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https://phys.org/news/2009-05-bird-songs-environment.html
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Answer: Both smallpox and COV 19 are novel diseases in their respective timelines. Both spread by inhaling infected droplets, albeit COV 19 is transmitted through aerosols and surfaces touched by infected people as well.
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