TEXT D Helium is a gas that is lighter than air. It is the second lightest of all gases. (Only hydrogen is lighter.) Helium is inert, which means it will not react chemically with anything. It cannot burn. It has no color, odor, or taste. The first people who saw evidence of helium saw it as a bright yellow line in spectroscopes they aimed at the sun during the eclipse of 1868. They knew that they were seeing something new, and that the chemical element that caused the line in the spectroscope was unknown on earth. The unknown element was named helium, from the Greek word helios, meaning "sun". Later, in 1895, the same brilliant yellow line was seen coming from a Norwegian mineral which produces a little helium by its radioactivity. The line in the spectroscope was identified as caused by the gas helium. This element is the only one that was discovered off the earth before it was discovered in the earth. Almost all commercial helium is produced in the United States. The plants for separating helium from natural gas are in Texas, Kansas, and New Mexico. Most of the natural gas supplies are controlled by the government and administered by the United States Bureau of Mines. The government is interested in helium because the gas has military uses. Helium has been used for some time to inflate blimps, weather balloons, advertising balloons, and toy balloons. Helium is valuable for filling balloons because it does not burn, as hydrogen does. This prevents dangerous explosions. Recent uses of helium are far more important than any of these familiar uses. Now helium gas may be the cooling gas, or coolant, pumped through the reactors of some nuclear power plants to keep them from getting too hot. It is also used in electric arc welding because the gas does not combine with hot metal. helium is used, too, in rocket fuel tanks as a means of maintaining pressure during flight.
　　76. The chemical element helium is named from the Greek word which means _____.
　　A) light air
　　B) yellow line
　　C) sun
　　D) earth
　　77. Helium is valuable to nuclear power plants because it _____.
　　A) is lighter than air
　　B) may be used as a cooling gas
　　C) does not combine with hot objects
　　D) does not burn