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Most electrification systems use overhead wires, but third rail is an option up to about 1,200 V. Third rail systems exclusively use DC distribution. The use of AC is not feasible because the dimensions of a third rail are physically very large compared with the skin depth that the alternating current penetrates to (0.3 millimetres or 0.012 inches) in a steel rail). This effect makes the resistance per unit length unacceptably high compared with the use of DC. Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.
What is physically more compact tird rail or overhead wires?
A: Third rail

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The passenger cabs are mechanically separated from the lift mechanism, thus allowing the elevator shafts to be used continuously while passengers board and embark from the cabs, as well as move through show scenes on various floors. The passenger cabs, which are automated guided vehicles or AGVs, move into the vertical motion shaft and lock themselves in before the elevator starts moving vertically. Multiple elevator shafts are used to further improve passenger throughput. The doorways of the top few "floors" of the attraction are open to the outdoor environment, thus allowing passengers to look out from the top of the structure.
What type of vehicles are the passenger cabs?
A: automated guided vehicles or AGVs

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As in much of Europe, the prosperity of Alsace came to an end in the 14th century by a series of harsh winters, bad harvests, and the Black Death. These hardships were blamed on Jews, leading to the pogroms of 1336 and 1339. In 1349, Jews of Alsace were accused of poisoning the wells with plague, leading to the massacre of thousands of Jews during the Strasbourg pogrom. Jews were subsequently forbidden to settle in the town. An additional natural disaster was the Rhine rift earthquake of 1356, one of Europe's worst which made ruins of Basel. Prosperity returned to Alsace under Habsburg administration during the Renaissance.
Why did Alsace decline has a prospering territory?
A: harsh winters, bad harvests, and the Black Death

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Richmond is home to the rapidly developing Virginia BioTechnology Research Park, which opened in 1995 as an incubator facility for biotechnology and pharmaceutical companies. Located adjacent to the Medical College of Virginia (MCV) Campus of Virginia Commonwealth University, the park currently[when?] has more than 575,000 square feet (53,400 m2) of research, laboratory and office space for a diverse tenant mix of companies, research institutes, government laboratories and non-profit organizations. The United Network for Organ Sharing, which maintains the nation's organ transplant waiting list, occupies one building in the park. Philip Morris USA opened a $350 million research and development facility in the park in 2007. Once fully developed, park officials expect the site to employ roughly 3,000 scientists, technicians and engineers.
What does the United Network for Organ Sharing do?
A:
maintains the nation's organ transplant waiting list