Answer: The mesosphere starts at 31 miles (50 km) and extends to 53 miles (85 km) high. The top of the mesosphere, called the mesopause, is the coldest part of Earth's atmosphere, with temperatures averaging about minus 130 degrees F (minus 90 C). This layer is hard to study
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Wilbur and Orville Wright were American inventors and pioneers of aviation. In 1903 the Wright brothers achieved the first powered, sustained and controlled airplane flight; they surpassed their own milestone two years later when they built and flew the first fully practical airplane.
Bacteria were first observed by the Dutch microscopist Antonie van Leeuwenhoek in 1676
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prevents backflow into the left atrium
