Problem: Stars, planets, and moons keep their atmospheres by gravitational attraction, and as such, atmospheres have no clearly delineated boundary: the density of atmospheric gas simply decreases with distance from the object. The Earth's atmospheric pressure drops to about 6998320000000000000♠3.2×10−2 Pa at 100 kilometres (62 mi) of altitude, the Kármán line, which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the Sun and the dynamic pressure of the solar winds, so the definition of pressure becomes difficult to interpret. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather. Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimetre.
What do Astrophysicists use to describe outer space beyond the karman line?
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Answer: number density


Problem: By the 20th century, the saloon, or lounge bar, had become a middle-class room[citation needed]—carpets on the floor, cushions on the seats, and a penny or two on the prices,[citation needed] while the public bar, or tap room, remained working class with bare boards, sometimes with sawdust to absorb the spitting and spillages (known as "spit and sawdust"), hard bench seats, and cheap beer[citation needed]. This bar was known as the four-ale bar from the days when the cheapest beer served there cost 4 pence (4d) a quart.[citation needed]
What socioeconomic class was associated with the lounge bar in the 20th century?
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Answer: middle-class


Problem: In China, urbanization increased as the population grew and as the division of labor grew more complex. Large urban centers, such as Nanjing and Beijing, also contributed to the growth of private industry. In particular, small-scale industries grew up, often specializing in paper, silk, cotton, and porcelain goods. For the most part, however, relatively small urban centers with markets proliferated around the country. Town markets mainly traded food, with some necessary manufactures such as pins or oil. Despite the xenophobia and intellectual introspection characteristic of the increasingly popular new school of neo-Confucianism, China under the early Ming dynasty was not isolated. Foreign trade and other contacts with the outside world, particularly Japan, increased considerably. Chinese merchants explored all of the Indian Ocean, reaching East Africa with the treasure voyages of Zheng He.
Under what rule was China not isolated?
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Answer:
the early Ming dynasty