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Seattle is one of the most ______ cities in the US.

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The factory was fined for discharging chemicals into the river.

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In Alaska, the ecotourism operators are designed to help minimise tourism ______, which may be more difficult to control as an independent traveler.

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Ecotourism ______  are important to the overall sustainability of the country's tourism industries.

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Texas is ______ to a number of eco-tour companies, and plenty of ranches and resorts.

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Species have               to climate changes throughout history.

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              is holidays and travel that involve causing less damage to the environment than usual and that make people aware of the need to protect the environment.