According to the author, what is the impracticability of using coal, oil shale and coal tars' as sources of energy due to?

Câu hỏi :

According to the author, what is the impracticability of using coal, oil shale and coal tars' as sources of energy due to?

A. They take a lot of time

B. They are expensive

C. They are non-renewable resources

D. They are scarce resources

* Đáp án

B

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