Comets(彗星)arepartsofoursolarsystem.Liketheplanets,theygoaroundthesun.
Butcometsarenotmadeofsolidrockslikeplanets.Acometisaballofdust,stones,andice.Manypeoplecallcometsdirtysnowballs.
The“snowball”maybeonlyafewmilesacross.Butwhenthesunheatsthe“snowball,”muchofitischangedtogases.Thegasesspreadoutandformthecomet’shead,whichmaybethousandsofmilesacross.
Acometmovesfastinitstriparoundthesun.Butwhenweseeit,thecometdoesnotseemtomove.That’sbecauseitissofaraway.Themoonmovesfast,butwhenyoulookatit,youcan’tseeanymotion(运动).That’sbecausethemoonisfaraway.
Eachyearastronomers(天文学家)discovernewcomets.Someofthemareseenonlyonce.Theymakeonetriparoundthesunandthengoawayoutintospace.Thesun’sgravitycannotholdthem.
Othercomets,likeHalley,keepreturning.Theyhavebeencapturedbythesun.Halley’searliestvisitwasprobably3,000yearsago.Itmaykeepreturningforanother3,000years.
Buteverytimeacometgoesaroundthesun,thecometlosespartofitself.Gasesanddustarepulledoutofthecomet.That’swhyHalleyisnowdimmer（暗淡的）thanitusedtobe.Nexttimeitvisitsus,in2062,itmaybeevendimmer.Eachvisititmaygetdimmeranddimmer,untilitfinallydisappears.
小题1:Whatisthispassagemostlyabout? |
| A．Cometsandthesun. |
| B．Whycometsarelikesnowballs. |
| C．Whodiscoveredthefirstcomet. |
| D．Factsaboutcomets. |
小题2:Howdoesthewriterhelpthereaderunderstandwhatacometlookslike?
| A．Bytellingaboutafamousone. |
| B．Byexplaininghowfarawayitis. |
| C．Bydescribinghowfastitmoves. |
| D．Bydescribingitasadirtysnowball. |
小题3:Whatisthemostlikelyreasonthewriterwrotethispassage?
| A．Togiveinformationaboutcomets. |
| B．Totellreadersaboutfamouscomets. |
| C．Toexplaintheimportanceofcomets. |
| D．Toencouragereaderstolookforcomets. |
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