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
In near future people relying on battery-powered devices to get around and do their work don't need to give up space in their packs to lots of extra batteries because ______.
A．their heavy packs can make electricity
B．battery-powered devices will be improved a lot
C．they typically carry heavy backpacks
D．global positioning system (GPS) receivers and night-vision goggles work themselves
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