填空题

During the 19th century, natural gas was used on a local scale because of the difficult situation in___________.

【参考答案】

transportation

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填空题

Often associated with crude oil, natural gas is principally made up of___________ and other hydrocarbon gases.

填空题

NGVs aren’t as roomy as gasoline cars because they have to give up cargo and trunk space to accommodate the___________.