ReadthefollowingpassageandmarktheletterA,B, C,orD onyouranswersheettoindicatethecorrectanswertoeachofthe questions.

Certainly nocreature in the sea is odder than the commonsea cucumber. All livingcreature,especiallyhumanbeings,havetheirpeculiarities,but everythingaboutthelittleseacucumberseemsunusual.Whatelsecan besaid aboutabizarreanimalthat,amongothereccentricities,eatsmud,feedsalmost continuouslydayandnightbutcanlivewithouteatingforlongperiods,and can be poisonous but is considered supremely edible by gourmets?

Forsomefiftymillionyears,despiteallitseccentricities,theseacucumber hassubsistedonitsdietofmud.Itisadaptableenoughtoliveattachedtorocks byitstubefeet,underrocksinshallowwater,oronthesurfaceofmudflats. Common in cool water on bothAtlantic and Pacific shores, it has the ability to such up mud or sand anddigest whatevernutrients are present.

Seacucumberscomeinavarietyofcolors,rangingfromblacktoreddish-browntosand-colorandnearlywhite.Oneformevenhasvividpurpletentacle. Usuallythecreaturesarecucumber-shaped-hencetheirname-andbecausethey aretypicallyrockinhabitants,thisshape,combinewithflexibility,enables themtosqueezeintocreviceswheretheyaresafefrompredatorsandocean currents.Althoughtheyhavevoraciousappetites,eatingdayandnight,seacucumbers havethecapacitytobecomequiescentandliveata lowmetabolicrate-feeding sparinglyornotatallforlongperiods,sothatthemarineorganismsthatprovide theirfoodhaveachancetomultiply.Ifitwerenotforthisfaculty,theywould devourallthefoodavailableinashorttimeandwouldprobablystarve themselves out of existence.

Butthemostspectacularthingabouttheseacucumberisthewayitdefends itself.Itsmajorenemiesarefishandcrabs,whenattacked,itsquirtsallits internalorgansintothewater.Italsocastsoffattachedstructuressuchas tentacles.Theseacucumberwilleviscerateandregenerateitselfifitisattached oreventouched;itwilldothesameifthesurroundingwatertemperatureistoo high or if the water becomestoo polluted.

The main idea of this passage is________.