You'veheardof3DprintingfromnewspaperandTVprograms．Itcan"print"inplastic，metal，nylon，andoverahundredothermaterials.3Dprintingtechnologyhasbeenaroundsincethe1980s，itwasnotwidelyuseduntiltheearly2010s．Thefirstworking3Dprinterwascreatedin1984byChuckHull．
3Dprintersusemanydifferenttypesoftechnologies，buttheyallshareonethingincommon：theycreateathree-dimensional（维度的）objectbybuildingitlayer（层），bycontinuinglayer，untilthewholeobjectiscompleted．Imagineabakerlaydowneachlayeroneatatimeuntilthewholecakeisformed.3Dprintingissomewhatsimilar，butjustabitstricter．Howcan3Dprintersworksowell？Themostimportantthing-aprogram，a3Dprinteralsoneedstohaveinstructionsforwhattoprintfromacomputer．Itneedsafileaswell．ThefilewhichisaComputerAidedDesign（CAD）paperiscreatedwiththeuseofa3Dmodelingprogram．Theprogramcreatesapaperthatissenttothe3Dprinter．Alongtheway，softwarepiecesthedesignintohundreds，ormorelikelythousandsoflayers．The3Dprinterbuildsuponelayeratatime，fromthebottomtothetop，untiltheobjectisdone．
3Dprintingscanbeusedformakingmanykindsofthingssuchasmachineparts，dailylifeobjects，evenhumanorgans（器官）usingaperson'sowncells．
Nowadays，Marc．DillonfoundsaprojectwhichdealswithmanyworldfamouspaintingslikeLeonardodaVinci'sMonaLisa，VanGogh'sSunflowerwith3Dtechnology．Thisprojecthelpstheblindpeopleenjoythefantasticartthroughtouching．Inordertoprovideagooduserexperience，these3Dpaintingshavespecialinstrumentswhichcantellpeoplewhattheytouch．
49．Thefirstworking3Dprinterwascreatedin___．
A.1980
B.1984
C.2001
D.2010
50．FromParagraph2，wecanknowthat___．
A．how a3D printer works
B．what a3D printing is
C．questionsofa3Dprinter
D．usesofa3D printer
51．Accordingtothepassage，___can help the blind know what they touch．
A．CADpaper
B．Machineparts．
C．Theworldfamouspaintings
D．Special instruments
52．Which of the following is true？___
A.3D p
