Solid geometry has enabled astronomers to calculate the positions of the heavenly bodies relative to one another.
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Themegaphone(microphone)makesthevoicesoundlouderbecauseitpointssoundwavesinonedirectionandkeepsthemfromspreadingoutinalldirections.
People who do not sleep enough tend to become irritable .
TheeconomyofDallas,Texas,isstronganddiversified.
Formerly,intheUnitedStates,manynursesworkedasprivatedutynursesratherthaninhospitals.
Eyespots, the most rudimentary eyes, are found in protozoan原生动物 flagellates 鞭毛虫 , flatworms 扁形虫 , and segmented worms片段 蠕虫 .