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.

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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

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