Mary often gets to school          .

A．ride her bike

B．by the bike

C．by a bike

D．on her bike

C.H2＝C.H－C.H＝C.H2在结构中存在的共轭体系是（）

A.p-π共轭

B.π-π共轭

C.σ-π共轭

D.σ-p共轭