Problem: Printed circuit board:

The hole walls for boards with two or more layers can be made conductive and then electroplated with copper to form plated-through holes.  These holes electrically connect the conducting layers of the PCB. For multi-layer boards, those with three layers or more, drilling typically produces a smear of the high temperature decomposition products of bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch. The de-smear process ensures that a good connection is made to the copper layers when the hole is plated through. On high reliability boards a process called etch-back is performed chemically with a potassium permanganate based etchant or plasma. The etch-back removes resin and the glass fibers so that the copper layers extend into the hole and as the hole is plated become integral with the deposited copper.

The back-etch removes what?
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A: unanswerable


Problem: As of December 2015[update], Barcelona has won 23 La Liga, 27 Copa del Rey, 11 Supercopa de España, three Copa Eva Duarte[note 2] and two Copa de la Liga trophies, as well as being the record holder for the latter four competitions. They have also won five UEFA Champions League, a record four UEFA Cup Winners' Cup, a shared record five UEFA Super Cup and a record three FIFA Club World Cup trophies. They also won a record three Inter-Cities Fairs Cup trophies, considered the predecessor to the UEFA Cup-Europa League.
What is the number of FIFA Club World  Cup trophies Barcelona has?
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Answer: three


Q: What is a question about this article? If the question is unanswerable, say "unanswerable".
Days of working time in Israel are Sunday through Thursday (for a five-day workweek), or Friday (for a six-day workweek). In observance of Shabbat, in places where Friday is a work day and the majority of population is Jewish, Friday is a "short day", usually lasting till 14:00 in the winter, or 16:00 in the summer. Several proposals have been raised to adjust the work week with the majority of the world, and make Sunday a non-working day, while extending working time of other days or replacing Friday with Sunday as a work day.
Friday is considered what?
A: short day


Context and question: The State Railways Administration of Uruguay (AFE) operates three commuter rail lines, namely the Empalme Olmos, San Jose and Florida. These lines operate to major suburban areas of Canelones, San José and Florida. Within the Montevideo city limits, local trains stop at Lorenzo Carnelli, Yatai (Step Mill), Sayago, Columbus (line to San Jose and Florida), Peñarol and Manga (line Empalme Olmos) stations. The historic 19th century General Artigas Central Station located in the neighbourhood of Aguada, six blocks from the central business district, was abandoned 1 March 2003 and remains closed. A new station, 500 metres (1,600 ft) north of the old one and part of the Tower of Communications modern complex, has taken over the rail traffic.
What does the AFE stand for?
Answer: The State Railways Administration of Uruguay


Question: Two chains of DNA twist around each other to form a DNA double helix with the phosphate-sugar backbone spiralling around the outside, and the bases pointing inwards with adenine base pairing to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align form two hydrogen bonds, whereas cytosine and guanine form three hydrogen bonds. The two strands in a double helix must therefore be complementary, with their sequence of bases matching such that the adenines of one strand are paired with the thymines of the other strand, and so on.:4.1
Is there an answer to this question: Due to the cause of the specificity of base pairing, what must be true of the two strands in a double helix?

Answer: The two strands in a double helix must therefore be complementary


Problem: ASCII:

The committee considered an eight-bit code, since eight bits (octets) would allow two four-bit patterns to efficiently encode two digits with binary-coded decimal. However, it would require all data transmission to send eight bits when seven could suffice. The committee voted to use a seven-bit code to minimize costs associated with data transmission. Since perforated tape at the time could record eight bits in one position, it also allowed for a parity bit for error checking if desired.:217, 236 § 5 Eight-bit machines (with octets as the native data type) that did not use parity checking typically set the eighth bit to 0.

What allowed for a parity bit for error chicking if needed?
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A:
perforated tape at the time could record eight bits in one position