TheAdventuresofTomSawyerand,especially,itssequence()provedthemselvestobethemilestoneintheAmericanliterature.
Ifyouarewalkingawayfromaclocktower，youwillhearthetickingoftheclockfadetoapoint________itcannotbeheard.
Lyingundertheskull,thehumanbraincontainsanaverageoftenbillionnervecellscalled______.