mechanism that automatically adjusts the internal pressure of his body. But （64）you can't bring a live whale into the laboratory for study, no one____（65）just how these things work.

　　51 A Their B It's C Theirs D Its

　　52 A strength B width C height D length

　　53 A eaten B crashed C towed（拖） D lifted

　　54 A distance B rate C voyage D sail

　　55 A picture B film C example D book

　　56 A off B of C above D over

　　57 A hit B seize C damage D kick

　　58 A fly to B jump on C dive to D crawl on

　　59 A In B On C By D At

　　60 A so B such C above D the same

　　61 A water B oxygen C energy D heat

　　62 A what B who C how D which

　　63 A it B he C she D there

　　64 A because of B due to C since D owing to

　　65 A communicates B knows C hears D develops