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

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

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