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It's thought that the accident was caused by human error.

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China has already been experimenting with ways to make its cities more ______ for the last two decades.

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New York has invested substantially in improving the ______ of its waterways in recent years.

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We will need new technologies to generate energy and use it in clean and safe ways, only from fully ______ sources.

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Vancouver is often considered to be one of the most ______ cities in the world.

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As we move toward 2050, we are facing the consequences of _____ urbanization and population growth.

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Telecommunications and cloud computing will ______ transportation for moving ideas and intellectual property.

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Copenhagen has been voted the most ______ city in the world several times.