Answer the question from the given passage. Your answer should be directly extracted from the passage, and it should be a single entity, name, or number, not a sentence.
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Question: Passage: Since then, and so far, general relativity has been acknowledged as the theory that best explains gravity. In GR, gravitation is not viewed as a force, but rather, objects moving freely in gravitational fields travel under their own inertia in straight lines through curved space-time – defined as the shortest space-time path between two space-time events. From the perspective of the object, all motion occurs as if there were no gravitation whatsoever. It is only when observing the motion in a global sense that the curvature of space-time can be observed and the force is inferred from the object's curved path. Thus, the straight line path in space-time is seen as a curved line in space, and it is called the ballistic trajectory of the object. For example, a basketball thrown from the ground moves in a parabola, as it is in a uniform gravitational field. Its space-time trajectory (when the extra ct dimension is added) is almost a straight line, slightly curved (with the radius of curvature of the order of few light-years). The time derivative of the changing momentum of the object is what we label as 'gravitational force'. Question: In what sense must you be observing the curvature of space-time?

Answer: global


Question: Passage: Many known complexity classes are suspected to be unequal, but this has not been proved. For instance P ⊆ NP ⊆ PP ⊆ PSPACE, but it is possible that P = PSPACE. If P is not equal to NP, then P is not equal to PSPACE either. Since there are many known complexity classes between P and PSPACE, such as RP, BPP, PP, BQP, MA, PH, etc., it is possible that all these complexity classes collapse to one class. Proving that any of these classes are unequal would be a major breakthrough in complexity theory. Question: What is an expression that can be used to illustrate the suspected inequality of complexity classes?

Answer: P ⊆ NP ⊆ PP ⊆ PSPACE


Question: Passage: The first recorded settlement in what is now Newcastle was Pons Aelius, a Roman fort and bridge across the River Tyne. It was given the family name of the Roman Emperor Hadrian, who founded it in the 2nd century AD. This rare honour suggests that Hadrian may have visited the site and instituted the bridge on his tour of Britain. The population of Pons Aelius at this period was estimated at 2,000. Fragments of Hadrian's Wall are still visible in parts of Newcastle, particularly along the West Road. The course of the 'Roman Wall' can be traced eastwards to the Segedunum Roman fort in Wallsend—the 'wall's end'—and to the supply fort Arbeia in South Shields. The extent of Hadrian's Wall was 73 miles (117 km), spanning the width of Britain; the Wall incorporated the Vallum, a large rearward ditch with parallel mounds, and was constructed primarily for defence, to prevent unwanted immigration and the incursion of Pictish tribes from the north, not as a fighting line for a major invasion. Question: What was the estimated population of Pons Aelius around the 2nd century?

Answer:
2,000