lesson91νppt课件
日期：2010-07-03 07:49
andexpensive返回目录Lesson91Whyarelightsignalsbetterthanelectricalsignals?返回目录Becauselightsignalscantravelalongerwaywithoutgettingweaker，heavy，Welcometojoinus!NextQuestionsabouttelephonesLesson91Lesson91Howdoesatelephonework?Thesoundwavesofyourvoicearechangedintoelectricalsignals，thesignalissentdownapairofwirestothereceivingphoneThenitisturnedintosoundagain返回目录Lesson91Whyweretelephonesexpensiveatthebeginning?Becauseitwasdifficultforpeopletoheareachotheroverlongdistances;andthewireswerethick，andtheirsignalisclearertooLesson91Whatisamobilephone?AmobilephoneisasmarttelephonewhichhasnowiresandcanbecarriedinyourpocketwhereveryougoLesson91Whatisthedifferencebetweenamobilephoneandanordinaryphone?First:Amobilephonehasnowires;Second:Youcantakeitwithyouwhereveryougo;Inaword:AmobilephoneisveryconvenienttouseLesson91Whatisacordlessphone?AcordlessphonealsohasnowiresItssignalcanbesentorreceivedinalimiteddistanceItisoftenusedintheofficeorthefamily返回目录Lesson91Ifyou，
查看全部