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This observation comes from the second to last sentence in the passage: As global demand rises, it is vital to produce more North American energy. The passage states that petroleum is a naturally-occurring liquid; therefore, you can eliminate the option that states that it is a man-made substance. Nowhere in the passage does it state the expense of petroleum; therefore, you can eliminate the option that states it is the cheapest source of energy. The passage states that petroleum accounts for 97 percent of the energy used for transportation; therefore, it cannot be the second most used energy source, it has to be the primary energy source for transportation.