Problem: Solar energy:

In the last two decades, photovoltaics (PV), also known as solar PV, has evolved from a pure niche market of small scale applications towards becoming a mainstream electricity source. A solar cell is a device that converts light directly into electricity using the photoelectric effect. The first solar cell was constructed by Charles Fritts in the 1880s. In 1931 a German engineer, Dr Bruno Lange, developed a photo cell using silver selenide in place of copper oxide. Although the prototype selenium cells converted less than 1% of incident light into electricity, both Ernst Werner von Siemens and James Clerk Maxwell recognized the importance of this discovery. Following the work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the crystalline silicon solar cell in 1954. These early solar cells cost 286 USD/watt and reached efficiencies of 4.5–6%. By 2012 available efficiencies exceed 20% and the maximum efficiency of research photovoltaics is over 40%.

In what year was the crystalline silicon solar cell constructed?
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A: 1954


Problem: In December 2008, the CTO of Blitz Games announced that it would bring stereoscopic 3D gaming and movie viewing to Xbox 360 and PlayStation 3 with its own technology. This was first demonstrated publicly on PS3 using Sony's own technology in January 2009 at the Consumer Electronics Show. Journalists were shown Wipeout HD and Gran Turismo 5 Prologue in 3D as a demonstration of how the technology might work if it is implemented in the future. Firmware update 3.30 officially allowed PS3 titles to be played in 3D, requiring a compatible display for use. System software update 3.50 prepared it for 3D films. While the game itself must be programmed to take advantage of the 3D technology, titles may be patched to add in the functionality retroactively. Titles with such patches include Wipeout HD, Pain, and Super Stardust HD.
Along with Wipeout HD, what game did the press view in 3D in early 2009?
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Answer: Gran Turismo 5 Prologue


Q: What is a question about this article? If the question is unanswerable, say "unanswerable".
Each team consists of a maximum of eleven players (excluding substitutes), one of whom must be the goalkeeper. Competition rules may state a minimum number of players required to constitute a team, which is usually seven. Goalkeepers are the only players allowed to play the ball with their hands or arms, provided they do so within the penalty area in front of their own goal. Though there are a variety of positions in which the outfield (non-goalkeeper) players are strategically placed by a coach, these positions are not defined or required by the Laws.
What is not excluded from the maximum amount of players?
A: unanswerable


Context and question: Prior Diego saw immediately one of the paramount reasons for the spread of the unorthodox movement: the representatives of the Holy Church acted and moved with an offensive amount of pomp and ceremony. On the other hand, the Cathars lived in a state of self-sacrifice that was widely appealing. For these reasons, Prior Diego suggested that the papal legates begin to live a reformed apostolic life. The legates agreed to change if they could find a strong leader. The prior took up the challenge, and he and Dominic dedicated themselves to the conversion of the Albigensians. Despite this particular mission, in winning the Albigensians over by persuasion Dominic met limited success, "for though in his ten years of preaching a large number of converts were made, it has to be said that the results were not such as had been hoped for."
Who did Prior Diego admire more than the Holy Church?
Answer: the Cathars


Question: Over the past two years,[clarification needed] dozens of rights-holders, including Sony Music, have sent complaints about Wikipedia.org directly to Google to have content removed.
Is there an answer to this question: Rights-holders have complained about Google to which organization?

Answer: unanswerable


Problem: Pitch (music):

It is still possible for two sounds of indefinite pitch to clearly be higher or lower than one another. For instance, a snare drum sounds higher pitched than a bass drum though both have indefinite pitch, because its sound contains higher frequencies. In other words, it is possible and often easy to roughly discern the relative pitches of two sounds of indefinite pitch, but sounds of indefinite pitch do not neatly correspond to any specific pitch. A special type of pitch often occurs in free nature when sound reaches the ear of an observer directly from the source, and also after reflecting off a sound-reflecting surface. This phenomenon is called repetition pitch, because the addition of a true repetition of the original sound to itself is the basic prerequisite.

Repetition pitch is caused by what phenomenon?
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A:
the addition of a true repetition of the original sound