Which of the following is NOT mentioned as a reason why...

Câu hỏi: Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 38 to 42.
The growth of cities, the construction of hundreds of new factories, and the spread of railroads in the United States before 1850 had increased the need for better illumination. But the lighting in American homes had improved very little over that of ancient times. Through the colonial period, homes were lit with tallow candles or with a lamp of the kind used in ancient Rome - a dish of fish oil or other animal or vegetable oil in which a twisted rag served as a wick. Some people used lard, but they had to heat charcoal underneath to keep it soft and burnable. The sperm whale provided superior burning oil, but this was expensive. In 1830 a new substance called "camphene" was patented, and it proved to be an excellent illuminant. But while camphene gave a bright light it too remained expensive, had an unpleasant odor, and also was dangerously explosive.
Between 1830 and 1850, it seemed that the only hope for cheaper illumination in the United States was the wider use of gas. In the 1840s, American gas manufacturers adopted improved British techniques for producing illuminating gas from coal. But the expense of piping gas to the consumer remained so high that until the mid-nineteenth century gas lighting was feasible only in urban areas, and only for public buildings for the wealthy. In 1854, a Canadian doctor, Abraham Gesner, patented a process for distilling a pitch like mineral found in New Brunswick and Nova Scotia that produced illuminating gas and an oil that he called "kerosene" (from "keros", the Greek word for wax, and "ene" because it resembled camphene). Kerosene, though cheaper than camphene, had an unpleasant odor, and Gesner never made his fortune from it. But Gesner had aroused a new hope for making illuminating oil from a product coming out of North American mines.
Which of the following is NOT mentioned as a reason why better lighting had become necessary by the mid-nineteenth century?
A. increases in the number of new factories
B. growth of cities
C. development of railroads
D. demand for better medical facilities