Q: What is a question about this article? If the question is unanswerable, say "unanswerable".
Linking the information back together is the key to this system. In the relational model, some bit of information was used as a "key", uniquely defining a particular record. When information was being collected about a user, information stored in the optional tables would be found by searching for this key. For instance, if the login name of a user is unique, addresses and phone numbers for that user would be recorded with the login name as its key. This simple "re-linking" of related data back into a single collection is something that traditional computer languages are not designed for.
What is a key used for?
A: uniquely defining a particular record

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To maintain a high constant body temperature is energy expensive – mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals – this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants. A herbivorous diet includes subtypes such as fruit-eating and grass-eating. An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract, because the proteins, lipids, and minerals found in meat require little in the way of specialized digestion. Plants, on the other hand, contain complex carbohydrates, such as cellulose. The digestive tract of an herbivore is therefore host to bacteria that ferment these substances, and make them available for digestion. The bacteria are either housed in the multichambered stomach or in a large cecum. The size of an animal is also a factor in determining diet type. Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high energy requirements and a high metabolic rate. Mammals that weigh less than about 18 oz (500 g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than 18 oz (500 g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).
Where are bacteria for digestion housed in an omnivore?
A: unanswerable

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Many Indo-Aryan languages have aspirated stops. Sanskrit, Hindi, Bengali, Marathi, and Gujarati have a four-way distinction in stops: voiceless, aspirated, voiced, and breathy-voiced or voiced aspirated, such as /p pʰ b bʱ/. Punjabi has lost breathy-voiced consonants, which resulted in a tone system, and therefore has a distinction between voiceless, aspirated, and voiced: /p pʰ b/.
Which distinction has Punjabi lost?
A: breathy-voiced consonants

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The 1973–74 season saw the arrival of Johan Cruyff, who was bought for a world record £920,000 from Ajax. Already an established player with Ajax, Cruyff quickly won over the Barcelona fans when he told the European press that he chose Barcelona over Real Madrid because he could not play for a club associated with Francisco Franco. He further endeared himself when he named his son Jordi, after the local Catalan Saint George. Next to champions like Juan Manuel Asensi, Carles Rexach and Hugo Sotil, he helped the club win the 1973–74 season for the first time since 1960, defeating Real Madrid 5–0 at the Bernabéu along the way. He was crowned European Footballer of the Year in 1973 during his first season with Barcelona (his second Ballon d'Or win; he won his first while playing for Ajax in 1971). Cruyff received this prestigious award a third time (the first player to do so) in 1974, while he was still with Barcelona.
 For what team did Cruyff refuse to play?
A:
Real Madrid