[Q]: Extract the answer to the question from the following context. Question: What happens when electrons and holes combine? Context: The structure of QD-LEDs used for the electrical-excitation scheme is similar to basic design of OLEDs. A layer of quantum dots is sandwiched between layers of electron-transporting and hole-transporting materials. An applied electric field causes electrons and holes to move into the quantum dot layer and recombine forming an exciton that excites a QD. This scheme is commonly studied for quantum dot display. The tunability of emission wavelengths and narrow bandwidth is also beneficial as excitation sources for fluorescence imaging. Fluorescence near-field scanning optical microscopy (NSOM) utilizing an integrated QD-LED has been demonstrated.
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[A]: forming an exciton
input: Please answer the following: Extract the answer to the question from the following context. Question: Who sung the song Single Ladies? Context: On September 13, 2009, during the 2009 MTV Video Music Awards while Taylor Swift was accepting her award for Best Female Video for "You Belong with Me", West went on stage and grabbed the microphone to proclaim that Beyonce's video for "Single Ladies (Put a Ring on It)", nominated for the same award, was "one of the best videos of all time". He was subsequently removed from the remainder of the show for his actions. When Beyonce later won the award for Best Video of the Year for "Single Ladies (Put a Ring on It)", she called Swift up on stage so that she could finish her acceptance speech. West was criticized by various celebrities for the outburst, and by President Barack Obama, who called West a "jackass". In addition, West's VMA disruption sparked a large influx of Internet photo memes with blogs, forums and "tweets" with the "Let you finish" photo-jokes. He posted a Tweet soon after the event where he stated, "Everybody wanna booooo me but I'm a fan of real pop culture... I'm not crazy y'all, I'm just real." He then posted two apologies for the outburst on his personal blog; one on the night of the incident, and the other the following day, when he also apologized during an appearance on The Jay Leno Show. After Swift appeared on The View two days after the outburst, partly to discuss the matter, West called her to apologize personally. Swift said she accepted his apology.
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output: Beyonce
Please answer this: Extract the answer to the question from the following context. Question: what is the fifth country mentioned? Context: Southern Europe's most emblematic climate is that of the Mediterranean climate, which has become a typically known characteristic of the area. The Mediterranean climate covers much of Portugal, Spain, Southeast France, Italy, Croatia, Albania, Montenegro, Greece, the Western and Southern coastal regions of Turkey as well as the Mediterranean islands. Those areas of Mediterranean climate present similar vegetations and landscapes throughout, including dry hills, small plains, pine forests and olive trees.
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Answer: Croatia
Problem: Extract the answer to the question from the following context. Question: Which group is the third-most prominent racial demographic in this public school system? Context: The Boston Public Schools enrolls 57,000 students attending 145 schools, including the renowned Boston Latin Academy, John D. O'Bryant School of Math & Science, and Boston Latin School. The Boston Latin School, established 1635, is the oldest public high school in the US; Boston also operates the United States' second oldest public high school, and its oldest public elementary school. The system's students are 40% Hispanic or Latino, 35% Black or African American, 13% White, and 9% Asian. There are private, parochial, and charter schools as well, and approximately 3,300 minority students attend participating suburban schools through the Metropolitan Educational Opportunity Council.

A: White
Problem: Given the question: Extract the answer to the question from the following context. Question: Who wrote a song? Context: An Ancient Chinese document from the 9th century referred to the northern Somali coast which was then called "Berbera" by Arab geographers in reference to the region's "Berber" (Cushitic) inhabitants as Po-pa-li. The first clear written reference of the sobriquet Somali, however, dates back to the 15th century. During the wars between the Sultanate of Ifat based at Zeila and the Solomonic Dynasty, the Abyssinian Emperor had one of his court officials compose a hymn celebrating a military victory over the Sultan of Ifat's eponymous troops.
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The answer is:
court officials
Problem: Extract the answer to the question from the following context. Question: When a class is not a set? Context: The second approach to the problem took as its base the notion of class, and defines a set as a class which belongs to other classes, while a proper class is defined as a class which does not belong to other classes. Under the ZermeloFraenkel approach, the axioms impede the construction of a set of all sets which do not belong to themselves. In contrast, under the von Neumann approach, the class of all sets which do not belong to themselves can be constructed, but it is a proper class and not a set.

A:
class of all sets which do not belong to themselves