Context: In addressing the question of who invented the incandescent lamp, historians Robert Friedel and Paul Israel list 22 inventors of incandescent lamps prior to Joseph Swan and Thomas Edison. They conclude that Edison's version was able to outstrip the others because of a combination of three factors: an effective incandescent material, a higher vacuum than others were able to achieve (by use of the Sprengel pump) and a high resistance that made power distribution from a centralized source economically viable.

Question: What made power distribution economically viable in Edison's lamp?
Answer: high resistance

Question: How many inventors came up with electric lamps before Thomas Edison?
Answer: 22

Question: How did Edison achieve a higher vacuum than other inventors?
Answer: by use of the Sprengel pump

Question: Which historians wrote about the advantages of Edison's lamp over other early versions?
Answer: Robert Friedel and Paul Israel

Question: How many inventors do historians list that did not invent the incandescent lamp?
Answer: Unanswerable

Question: What made power distribution economically non-viable in Edison's lamp?
Answer: Unanswerable

Question: What four factors made Edison's version economically viable?
Answer: Unanswerable

Question: Which three historians wrote the conclusion that Edison's version was more viable than other early versions?
Answer: Unanswerable