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
Natural gas is recognized as the most economical energy source______.

【参考答案】

F
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单项选择题
Which of the following is closest in meaning to the word sparked appearing in paragraph 2
A．flashed
B．stimulated
C．changed
D．ended
填空题
B
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