An ADult girAFFE’s hEAD is ABout six FEEt ABovE its hEArt. this mEAns thAt to{{u}} (67) {{/u}}Enough BlooD up to thE BrAin thE CirCulAtory{{u}} (68) {{/u}}must BE strong Enough to kEEp thE BlooD At vEry high prEssurE. Biologists hAvE known For somE timE thAt girAFFEs solvE this proBlEm By hAving{{u}} (69) {{/u}}high BlooD prEssurE, ABout{{u}} (70) {{/u}}thAt oF humAn BEings.But An intErnAtionAl tEAm oF Biologists BEgAn to{{u}} (71) {{/u}}ABout this. iF girAFFEs hAvE suCh high BlooD prEssurE, thEy shoulD hAvE A{{u}} (72) {{/u}}proBlEm with swElling in thEir lEgs AnD FEEt. why Don’t girAFFEs hAvE swollEn FEEt girAFFEs shoulD hAvE{{u}} (73) {{/u}}proBlEm, too.EvEry timE thEy BEnD hEADs{{u}} (74) {{/u}}to Drink, thE BlooD shoulD{{u}} (75) {{/u}}to thEir hEADs AnD hAvE A hArD timE{{u}} (76) {{/u}}BACk up (whEn thE hEAD is Down) to thE hEArt. how ComE girAFFEs Don’t BlACk out whEn thEy Drink thE AnswEr to thE{{u}} (77) {{/u}}FEEt proBlEm, thE rEsEArChErs FounD, is thAt girAFFEs hAvE{{u}} (78) {{/u}}thE rEsEArChErs CAll A "nAturAl Anti-grAvity suit". it{{u}} (79) {{/u}}out thAt thE skin AnD othEr{{u}} (80) {{/u}}in thEir lEgs AnD FEEt ArE{{u}} (81) {{/u}} stiFFEr AnD toughEr thAn thosE oF othEr{{u}} (82) {{/u}}.As A rEsult, thE BlooD vEssEls in thE lEg CAnnot swEll. thErEForE, thE BlooD hAs nowhErE to go But BACk to thE hEArt. whAt ABout BlooD rushing to thE hEAD{{u}} (83) {{/u}}thE girAFFE BEnDs Down to Drink thE rEsEArChErs FounD thAt thE girAFFE’s jugulAr vEin, whiCh{{u}} (84) {{/u}}BlooD From thE hEAD BACk to thE hEArt, hAs lots oF onE-wAy vAlvEs in it. in thE girAFFE’s nECk, thErE ArE lots oF musClEs thAt FlEx AnD rElAx rEpEAtEDly As thE AnimAl movEs its hEAD AnD suCks{{u}} (85) {{/u}} Drinking wAtEr.By squEEzing thE vAlvED jugulAr vEin, thEy{{u}} (86) {{/u}}BlooD moving BACk to thE hEArt EvEn whilE thE AnimAl is Drinking.

A．up
B．towArD
C．Down
D．AsiDE