单选题：Electdc Backpack 　　Backpacks are convenient．They can hold yo

　　Electdc Backpack 　　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 backpacks might also power your MP3 1 player，keep your cell phone running，and maybe even light your way home．
　　Lawrence C．Rome and his colleagues from the University of Pennsylvania in Philadelphia2 and the Marine Biological Laboratory in Woods Hole3，Mass．4，have invented a backpack that makes electricity from energy produced while its wcarer walks．In military actions．search—and—rescue operations，and scientific field studies，people rely increasingly on cell phones，global positioning system(GPS)5 receivers，night—vision goggles，and other battery—powered devices to get around and do their work．The backpack，s electricity—generating feature could dramatically reduce the amount of a wearer，s load now devoted to spare batteries，report Rome and his colleagues in the Sept．9 Science6．
　　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 gear meehanism converts vertical movements of the pack to rotary motions of an electrieal generator，producing up to 7．4 watts．
　　Unexpectedly，tests showed that wearers of the new backpack alter their gaits in response to the pack’s oscillations，so that they carry loads more comfortably and with less effort than they do ordinary backpacks．Because of that surprising advantage，Rome plans to commercialize botb electric and uon—electric versions of the backpack．
　　The backpack could be especially useful for soldiers，scientists，mountaineers，and emergency workers who typically carry heavy backpacks．For the rest of HS，power—generating backpacks could make it possible to walk，play rideo 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! 根据以上材料回答下面问题：Backpacks are convenient because A．they can be very large
B．they can hold as many things as you want to carr
C．your hands are freed to do other thing
D．you do not have to carry things with yo