Article: 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.

Question: What depth does the alternating current penetrate in a steel rail?
Ans: skin depth


Here is a question about this article: Symptoms of mild zinc deficiency are diverse. Clinical outcomes include depressed growth, diarrhea, impotence and delayed sexual maturation, alopecia, eye and skin lesions, impaired appetite, altered cognition, impaired host defense properties, defects in carbohydrate utilization, and reproductive teratogenesis. Mild zinc deficiency depresses immunity, although excessive zinc does also. Animals with a diet deficient in zinc require twice as much food in order to attain the same weight gain as animals given sufficient zinc.
What is the answer to this question: What is required for animals with zinc deficiency to gain the same weight as animals with sufficient zinc?
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So... twice as much food


The problem: Answer a question about this article:
The context for the rise of the public sphere was the economic and social change commonly associated with the Industrial Revolution: "economic expansion, increasing urbanization, rising population and improving communications in comparison to the stagnation of the previous century"." Rising efficiency in production techniques and communication lowered the prices of consumer goods and increased the amount and variety of goods available to consumers (including the literature essential to the public sphere). Meanwhile, the colonial experience (most European states had colonial empires in the 18th century) began to expose European society to extremely heterogeneous cultures, leading to the breaking down of "barriers between cultural systems, religious divides, gender differences and geographical areas".
Was the variety of goods available to consumers increased or decreased by the economic and social change of the Industrial Revolution?
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The answer: increased


Problem: Please answer a question about the following article about The Blitz:
The military effectiveness of bombing varied. The Luftwaffe dropped around 45,000 short tons (41,000 t) of bombs during the Blitz disrupting production and transport, reducing food supplies and shaking the British morale. It also helped to support the U-Boat blockade by sinking some 58,000 long tons (59,000 t) of shipping destroyed and 450,000 long tons (460,000 t) damaged. Yet, overall the British production rose steadily throughout this period although there were significant falls during April 1941, probably influenced by the departure of workers of Easter Holidays according to the British official history. The British official history on war production noted the great impact was upon the supply of components rather than complete equipment. In aircraft production, the British were denied the opportunity to reach the planned target of 2,500 aircraft in a month, arguably the greatest achievement of the bombing, as it forced the dispersal of industry. In April 1941, when the targets were British ports, rifle production fell by 25%, filled-shell production by 4.6%, and in smallarms production 4.5% overall. The strategic impact on industrial cities was varied; most took from 10–15 days to recover from heavy raids, although Belfast and Liverpool took longer. The attacks against Birmingham took war industries some three months to recover fully from. The exhausted population took three weeks to overcome the effects of an attack.
The Luftwaffe used how many short tons of bombs during the Blitz?
A: 45,000 short tons


Question: Read this and answer the question

Some advocates for Tibet, Darfur, and the spiritual practice Falun Gong, planned to protest the April 9 arrival of the torch in San Francisco. China had already requested the torch route in San Francisco be shortened. On April 7, 2008, two days prior to the actual torch relay, three activists carrying Tibetan flags scaled the suspension cables of the Golden Gate Bridge to unfurl two banners, one saying "One World, One Dream. Free Tibet", and the other, "Free Tibet '08". Among them was San Francisco resident Laurel Sutherlin, who spoke to the local TV station KPIX-CBS5 live from a cellphone, urging the International Olympic Committee to ask China not to allow the torch to go through Tibet. "Sutherlin said he was worried that the torch's planned route through Tibet would lead to more arrests and Chinese officials would use force to stifle dissent." The three activists and five supporters face charges related to trespassing, conspiracy and causing a public nuisance.

How many activists may be charged with crimes for the bridge stunt?
Answer: three


There are also open compression formats like Opus and Vorbis that are available free of charge and without any known patent restrictions. Some of the newer audio compression formats, such as AAC, WMA Pro and Vorbis, are free of some limitations inherent to the MP3 format that cannot be overcome by any MP3 encoder.
Being open formats means that Opus and Vorbis can be used without what?
any known patent restrictions