单项选择题

The backpack's electricity comes from ______.

A．the gears with battery-powered devices
B．the motion of the whole backpack while one walks
C．a mechanism with gears
D．an electrical generator
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单项选择题
The first paragraph hints (暗示) ______.
A．backpacks are convenient
B．someday backpacks can generate electricity
C．MP3 players and cell phones can also be put into a backpack
D．backpacks hold things for people so as to free their hands to do other things.
单项选择题
The electricity-generating packs are likely to be dangerous to us ______.
A．while lighting his way
B．when playing video games, watching TV, and listening to music, all at the same time
C．while crossing the street
D．at the time when creating powers
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