Q: What is a question about this article? If the question is unanswerable, say "unanswerable".
The majority of the world's airports are non-towered, with no air traffic control presence. However, at particularly busy airports, or airports with other special requirements, there is an air traffic control (ATC) system whereby controllers (usually ground-based) direct aircraft movements via radio or other communications links. This coordinated oversight facilitates safety and speed in complex operations where traffic moves in all three dimensions. Air traffic control responsibilities at airports are usually divided into at least two main areas: ground and tower, though a single controller may work both stations. The busiest airports also have clearance delivery, apron control, and other specialized ATC stations.
What describes most clearance delivery systems?
A: unanswerable

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In 2004, NIST researchers presented evidence that an isotropic non-crystalline metallic phase (dubbed "q-glass") could be grown from the melt. This phase is the first phase, or "primary phase," to form in the Al-Fe-Si system during rapid cooling. Interestingly, experimental evidence indicates that this phase forms by a first-order transition. Transmission electron microscopy (TEM) images show that the q-glass nucleates from the melt as discrete particles, which grow spherically with a uniform growth rate in all directions. The diffraction pattern shows it to be an isotropic glassy phase. Yet there is a nucleation barrier, which implies an interfacial discontinuity (or internal surface) between the glass and the melt.
What shows that there is an inner surface between glass and melt?
A: nucleation barrier

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In 1933, Herbert Chapman, wanting his players to be more distinctly dressed, updated the kit, adding white sleeves and changing the shade to a brighter pillar box red. Two possibilities have been suggested for the origin of the white sleeves. One story reports that Chapman noticed a supporter in the stands wearing a red sleeveless sweater over a white shirt; another was that he was inspired by a similar outfit worn by the cartoonist Tom Webster, with whom Chapman played golf. Regardless of which story is true, the red and white shirts have come to define Arsenal and the team have worn the combination ever since, aside from two seasons. The first was 196667, when Arsenal wore all-red shirts; this proved unpopular and the white sleeves returned the following season. The second was 200506, the last season that Arsenal played at Highbury, when the team wore commemorative redcurrant shirts similar to those worn in 1913, their first season in the stadium; the club reverted to their normal colours at the start of the next season. In the 200809 season, Arsenal replaced the traditional all-white sleeves with red sleeves with a broad white stripe.
In what year was Arsenal founded?
A: unanswerable

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There are also three Deputy Chiefs of the Defence Staff with particular remits, Deputy Chief of the Defence Staff (Capability), Deputy CDS (Personnel and Training) and Deputy CDS (Operations). The Surgeon General, represents the Defence Medical Services on the Defence Staff, and is the clinical head of that service.
Who is the leader of the Deputy CDS?
A:
unanswerable