单项选择题

A．Twenty years.
B．Sixty years.
C．Forty years.
D．Eighty year.
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单项选择题
A．It’s environmentally friendly.B．It’s more efficient than other alternative fuels.C．It doesn’t destroy the ozone.D．It’s not economical to mass-produce.
A．It’s environmentally friendly.
B．It’s more efficient than other alternative fuels.
C．It doesn’t destroy the ozone.
D．It’s not economical to mass-produce.
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