填空题 A over the past 50 years
B beneath the earth surface
C by more than 50 percent
D for more than four decades
E as a raw material
F for families

填空题 Natural gas is stored deep______.

填空题 Natural gas is recognized as the most economical energy source______.

填空题 When manufacturing many products, people commonly use natural gas______.

填空题 It is estimated that by 2025 that natural gas demand in the United States will increase______.