Backpacks are convenient. They can hold your books, your lunch, and a change of clothes, leaving your hands free to do other things. Someday, if you don't mind carrying a heavy load, your backpack might also power your MP3 player, keep your cell phone running, and maybe even light your way home.
Scientists from the University of Pennsylvania in Philadelphia and the Marine Biological Laboratory in Woods Hole, Mass., have invented a backpack that makes electricity from energy produced while its wearer walks.
The backpack's electricity-creating powers depend on springs used to hang a cloth pack from its metal frame. The frame. sits against the wearer's back, and the whole pack moves up and down as the person walks. A mechanism with gears collects energy from this motion and transfers it to an electrical generator.
Surprisingly, the researchers found, people walk differently when they wear the springy packs. As a result, wearers use less energy than when lugging regular backpacks. Also, the way the new packs ride on wearers' backs makes them more comfortable than standard packs, the inventors say.
The backpack could be especially useful for soldiers, scientists, mountaineers, and emergency workers who typically carry heavy backpacks. These people often rely on global positioning system (GPS) receivers, night-vision goggles, arid other battery-powered devices to get around and do their work. Because the pack can make its own electricity, users don't need to give up space in their packs to lots of extra batteries.
For the rest of us, power-generating backpacks could make it possible to walk, play video games, watch TV, and listen to music, all at the same time. Electricity-generating packs aren't on the market yet, but if you do get one eventually, just make sure to look both ways before crossing the street!
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.