Only electric vehicles are capable of meeting a "zero-emission" standard set by California, New York and Massachusetts.

CNN: Natural gas vehicles on a roll in U.S.

Long-term regulations were needed to phase out carbon emissions in key areas - by reducing vehicle emissions and work towards "a zero-emission vehicle standard across Europe".

BBC: RELATED BBC SITES

Each standard is the simple reciprocal of an associated CO2 emission reduction scenario.

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