FormAl EConomiC ForECAsting is usuAlly BAsED on A{{u}} (1) {{/u}}thEory As to how thE EConomy works. somE thEoriEs ArE CompliCAtED, AnD thEir AppliCAtion rEquirEs An ElABorAtE{{u}} (2) {{/u}}oF CAusE AnD EFFECt. othErs ArE rElAtivEly simplE,{{u}} (3) {{/u}}most DEvElopmEnts in thE EConomy to onE or two BAsiC FACtors. mAny EConomists, For ExAmplE, BEliEvE thAt ChAngEs in thE supply oF monEy{{u}} (4) {{/u}}thE rAtE oF growth oF gEnErAl BusinEss ACtivity. othErs{{u}} (5) {{/u}}A CEntrAl rolE to invEstmEnt in nEw FACilitiEs-- housing, inDustriAl plAnts, highwAys, AnD so Forth. in thE unitED stAtEs, whErE ConsumErs{{u}} (6) {{/u}}suCh A lArgE shArE oF EConomiC ACtivity, somE EConomy BEliEvE thAt ConsumEr DECisions to{{u}} (7) {{/u}}or sAvE proviDE thE prinCipAl{{u}} (8) {{/u}}to thE FuturE CoursE oF thE EntirE EConomy. oBviously thE thEory thAt A ForECAstEr AppliEs is oF{{u}} (9) {{/u}}importAnCE to thE ForECAsting proCEss; it{{u}} (10) {{/u}}his linE oF invEstigAtion, thE stAtistiCs hE will rEgArD As most importAnt, AnD mAny oF thE tEChniquEs hE will Apply. Although EConomiC thEory mAy DEtErminE thE gEnErAl{{u}} (11) {{/u}}oF A ForECAst, juDgmEnt Also oFtEn plAys An importAnt rolE.A、ForECAstEr mAy DECiDE thAt thE CirCumstAnCEs oF thE momEnt ArE{{u}} (12) {{/u}}AnD thAt A ForECAst proDuCED By thE{{u}} (13) {{/u}}stAtistiCAl mEthoDs shoulD BE moDiFiED to tAkE ACCount oF spECiAl CurrEnt CirCumstAnCEs. this is pArtiCulArly nECEssAry whEn somE EvEnt outsiDE thE usuAl run oF EConomiC ACtivity hAs An A{{u}} (14) {{/u}}EConomiC EFFECt. For ExAmplE, ForECAsts oF 1987 EConomiC ACtivity in thE unitED stAtEs wErE morE ACCurAtE whEn thE AnAlyst CorrECtly ForEsAw thAt thE ExChAngE vAluE oF thE DollAr woulD{{u}} (15) {{/u}}shArply During thE yEAr thAt ConsumEr spEnDing woulD slACkEn, AnD thAt{{u}} (16) {{/u}}rAtEs woulD risE only moDErAtEly. nonE oF thEsE ConClusions FollowED{{u}} (17) {{/u}}purEly EConomiC AnAlysis; thEy All rEquirED juDgmEnt As to FuturE DECisions{{u}} (18) {{/u}}, An EConomist mAy DECiDE to ADjust An EConomiC ForECAst thAt wAs mADE By trADitionAl mEthoDs to tAkE ACCount oF othEr uniquE{{u}} (19) {{/u}}; hE mAy, For ExAmplE, DECiDE thAt ConsumErs will{{u}} (20) {{/u}}thEir spEnDing pAttErns BECAusE oF spECiAl CirCumstAnCEs suCh As rising priCE oF imports or FEAr oF thrEAtEnED shortAgEs.

A．quit
B．AltEr
C．prEsErvE
D．invEnt