题目内容：
回答题ElectriBackpack
Backpacksare convenient.They can holyour books,your lunch,anachange ofclothes,leaving your hands free to do other things.Someday,if youdont mindcarrying heavy load,your backpacks might also power your MP31player,keep yourcell phone running,anmaybe even light your wayhome.
LawrenceC.Rome anhis colleagues from the University of Pennsylvaniin Philadelphia2anthe Marine Biological Laboratory in WoodsHole3,Mass.4,have inventeabackpack that makes electricity from energyproducewhile its wearer walks.Inmilitary actions,search-and-rescueoperations,anscientififieldstudies,people rely increasingly on cellphones,global positioning system (GPS)5 receivers,night-vision goggles,andother battery poweredevices to get aroundando their work.The backpackselectricity- generating feature coulddramatically reduce the amount of awearer’s loanow devoteto spare batteries,report Rome anhis colleaguesinthe Sept.9 Science6.
Thebackpacks electricity-creating powers depenonsprings useto hang clothpack from its metal frame.The frame sits againstthe wearer’s back,anthe whole pack moves upandown as the person walks.Agear mechanism converts vertical movements of thepack to rotary motions of anelectrical generator,producing up to 7.4 watts.
Unexpectedly,testsshowedthat wearers of the new backpack alter their gaits in response to thepacksoscillations,so that they carry loads more comfortably anwith lesseffortthan they do ordinary backpacks.Because of that surprising advantage,Romeplans to commercialize both electriannon-electriversionsof the backpack.
The backpackcoulbe especially useful orsoldiers,scientists,mountaineers,anemergencyworkers who typically carryheavy backpacks.For the rest of us,power-generatingbackpacks coulmake itpossible to walk,play video games,watch TV,anlisten tomusic,all at the sametime.Electricity-generating packs arent on the marketyet,but if you do get one eventually,just make sure to look both ways beforecrossing thestreet!
Backpacks are convenient because they can A．change your gaits.
B．reduce your load.
C．free your hands.
D．control your MP3 player.
参考答案：
答案解析：