All of these alternatives are carbon-based fuels（）molecules are smaller and simpler than those of gasoline.

A.general
B.generation
C.generate
D.generous

A.twice as expensive as
B.twice as more expensive as
C.as twice more expensive as
D.as twice expensive as

A.three times as big as
B.three times as bigger as
C.as three times big as
D.as three times gigger as