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Ha Long Bay is the product of geological ______ that lasted more than 500 million years.

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The national park was established to ______ the ecosystem of the limestone forests.

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Scientists have been studying the elements that affect the develop of the native species.

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The red squirrel in England is in danger of ______.

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One of the factors that cause a species to become endangered is habitat ______.

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The ______ living by the river has been uprooted and then transported away.

The new varieties of rice grow well ______ in poor soil.

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Scientists have been studying different ecologies ______ the 20^th century.