Problem: Railway electrification system:

In the UK, the London, Brighton and South Coast Railway pioneered overhead electrification of its suburban lines in London, London Bridge to Victoria being opened to traffic on 1 December 1909. Victoria to Crystal Palace via Balham and West Norwood opened in May 1911. Peckham Rye to West Norwood opened in June 1912. Further extensions were not made owing to the First World War. Two lines opened in 1925 under the Southern Railway serving Coulsdon North and Sutton railway station. The lines were electrified at 6.7 kV 25 Hz. It was announced in 1926 that all lines were to be converted to DC third rail and the last overhead electric service ran in September 1929.

What was the cause of lines not being extended?
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A: the First World War

Problem: Successful cultivation of cotton requires a long frost-free period, plenty of sunshine, and a moderate rainfall, usually from 600 to 1,200 mm (24 to 47 in). Soils usually need to be fairly heavy, although the level of nutrients does not need to be exceptional. In general, these conditions are met within the seasonally dry tropics and subtropics in the Northern and Southern hemispheres, but a large proportion of the cotton grown today is cultivated in areas with less rainfall that obtain the water from irrigation. Production of the crop for a given year usually starts soon after harvesting the preceding autumn. Cotton is naturally a perennial but is grown as an annual to help control pests. Planting time in spring in the Northern hemisphere varies from the beginning of February to the beginning of June. The area of the United States known as the South Plains is the largest contiguous cotton-growing region in the world. While dryland (non-irrigated) cotton is successfully grown in this region, consistent yields are only produced with heavy reliance on irrigation water drawn from the Ogallala Aquifer. Since cotton is somewhat salt and drought tolerant, this makes it an attractive crop for arid and semiarid regions. As water resources get tighter around the world, economies that rely on it face difficulties and conflict, as well as potential environmental problems. For example, improper cropping and irrigation practices have led to desertification in areas of Uzbekistan, where cotton is a major export. In the days of the Soviet Union, the Aral Sea was tapped for agricultural irrigation, largely of cotton, and now salination is widespread.
What is the optimum amount of rainfall for cotton growth?
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Answer: 600 to 1,200 mm

Q: What is a question about this article? If the question is unanswerable, say "unanswerable".
Standard Chinese (Mandarin) has stops and affricates distinguished by aspiration: for instance, /t t/, /ts ts/. In pinyin, tenuis stops are written with letters that represent voiced consonants in English, and aspirated stops with letters that represent voiceless consonants. Thus d represents /t/, and t represents /t/.
Tenuis stops have letters that are representative of English voiced consonant in what?
A: pinyin

Context and question: In the 2010s, American jurisdictions have experienced a shortage of lethal injection drugs, due to anti-death penalty advocacy and low production volume. Hospira, the only U.S. manufacturer of sodium thiopental, stopped making the drug in 2011. The European Union has outlawed the export of any product that could be used in an execution; this has prevented executioners from using EU-manufactured anesthetics like propofol which are needed for general medical purposes. Another alternative, pentobarbital, is also only manufactured in the European Union, which has caused the Danish producer to restrict distribution to U.S. government customers.
What death penalty drug ceased to be manufactured in 2011?
Answer: sodium thiopental

Question: Thousands of fonts exist on the market, but fewer than a dozen fontssometimes described as "pan-Unicode" fontsattempt to support the majority of Unicode's character repertoire. Instead, Unicode-based fonts typically focus on supporting only basic ASCII and particular scripts or sets of characters or symbols. Several reasons justify this approach: applications and documents rarely need to render characters from more than one or two writing systems; fonts tend to demand resources in computing environments; and operating systems and applications show increasing intelligence in regard to obtaining glyph information from separate font files as needed, i.e., font substitution. Furthermore, designing a consistent set of rendering instructions for tens of thousands of glyphs constitutes a monumental task; such a venture passes the point of diminishing returns for most typefaces.
Is there an answer to this question: What are most of the fonts on the market called?

Answer: unanswerable

Problem: Hard rock:

In the early 1970s the Rolling Stones developed their hard rock sound with Exile on Main St. (1972). Initially receiving mixed reviews, according to critic Steve Erlewine it is now "generally regarded as the Rolling Stones' finest album". They continued to pursue the riff-heavy sound on albums including It's Only Rock 'n' Roll (1974) and Black and Blue (1976). Led Zeppelin began to mix elements of world and folk music into their hard rock from Led Zeppelin III (1970) and Led Zeppelin IV (1971). The latter included the track "Stairway to Heaven", which would become the most played song in the history of album-oriented radio. Deep Purple continued to define hard rock, particularly with their album Machine Head (1972), which included the tracks "Highway Star" and "Smoke on the Water". In 1975 guitarist Ritchie Blackmore left, going on to form Rainbow and after the break-up of the band the next year, vocalist David Coverdale formed Whitesnake. 1970 saw The Who release Live at Leeds, often seen as the archetypal hard rock live album, and the following year they released their highly acclaimed album Who's Next, which mixed heavy rock with extensive use of synthesizers. Subsequent albums, including Quadrophenia (1973), built on this sound before Who Are You (1978), their last album before the death of pioneering rock drummer Keith Moon later that year.

What two hard rock songs are featured on Deep Purple's Machine Head album?
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A:
"Highway Star" and "Smoke on the Water"