The problem: Answer a question about this article:
Bitumen was used in early photographic technology. In 1826 or 1827, it was used by French scientist Joseph Nicephore Niepce to make the oldest surviving photograph from nature. The bitumen was thinly coated onto a pewter plate which was then exposed in a camera. Exposure to light hardened the bitumen and made it insoluble, so that when it was subsequently rinsed with a solvent only the sufficiently light-struck areas remained. Many hours of exposure in the camera were required, making bitumen impractical for ordinary photography, but from the 1850s to the 1920s it was in common use as a photoresist in the production of printing plates for various photomechanical printing processes.[not in citation given]
What feature of bitumen use in photography made it impractical?
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The answer: hours of exposure

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Chopin's music remains very popular and is regularly performed, recorded and broadcast worldwide. The world's oldest monographic music competition, the International Chopin Piano Competition, founded in 1927, is held every five years in Warsaw. The Fryderyk Chopin Institute of Poland lists on its website over eighty societies world-wide devoted to the composer and his music. The Institute site also lists nearly 1,500 performances of Chopin works on YouTube as of January 2014.
When was the International Chopin Piano Competition established?
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The answer: 1927

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In Buddhist practice, it is said that while samatha meditation can calm the mind, only vipassana meditation can reveal how the mind was disturbed to start with, which is what leads to insight knowledge (jnana; Pali nana) and understanding (prajna Pali panna), and thus can lead to nirvana (Pali nibbana). When one is in jhana, all defilements are suppressed temporarily. Only understanding (prajna or vipassana) eradicates the defilements completely. Jhanas are also states that Arahants abide in order to rest.
What is the term for understanding?
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The answer: prajna

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In the example above, the identity and the rotations constitute a subgroup R = {id, r1, r2, r3}, highlighted in red in the group table above: any two rotations composed are still a rotation, and a rotation can be undone by (i.e. is inverse to) the complementary rotations 270 for 90, 180 for 180, and 90 for 270 (note that rotation in the opposite direction is not defined). The subgroup test is a necessary and sufficient condition for a subset H of a group G to be a subgroup: it is sufficient to check that g1h H for all elements g, h H. Knowing the subgroups is important in understanding the group as a whole.d["]
What essential condition must be met for a subset of a group to be a subgroup?
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The answer:
The subgroup test