题目内容：
根据以下资料，回答题。LightningflashedthroughthedarknessoverSibson’sbedroomskylight（天窗）.Sibsonwasshakenbyaclapofthunder____21___heknewwhatwashappening.Thestormhadmoveddirectly_____22____histwo-storywoodenhouse.Thenheheardthesmokealarmbeeping.Sibsonrusheddownthestairsbarefootto23;heopenedthedoortothebasement（地下室）,andflames24out.Sibsonranbackupstairstocall911fromhisbedroom.“Ifelt25becausetheroomhadaseparateoutdoorstairway,”heexplains.
Butthephonedidn’twork,andwhenhetriedtogodowntheoutdoorstairway,hewas26byawallofflames.Sibsonrealizedhewastrapped（困住）.
Sibson’shousewasthreekilometers27themainroadandwassowellhiddenbytreesthatheknewcallingforhelpwouldbe28.
UpahillnearbylivedSibson’sneighbor,Huggons.Hewaslyinginbedwhensomethinglikeasmokealarm29hisears.Hejumpedoutofbed,tookhis30andflashlight,andheadeddownthehillsidetowardthe31.Thatwaswhenhesawtherollingheavysmoke.
Huggonsdialed911,andtheoperatorwarnedhimnotto32thehouse.ButHuggonssaid,“ThereisnowayIamgoingtolistentoSibson33anddieinthatfire.”
“Anyonethere?”Huggonscalledout.Thenheheard“Help!I’mtrapped!”comingfromthesecond-floorbalcony（阳台）.Heenteredthehouse,butsoonhadtorunbacktocatchhis34.
Afteronemore35insidethehouse,Huggonsgaveupand36aroundback.
Thewindpartedthesmokejust37forhimtocatchsightofSibson.Buttherewasnowaytogettohim.He38theflashlightintothewoodsandnoticedaladder.Hetookitovertothebalconyand39Sibsondownjustasthesecondfloorofthehousefelloff.
Sibsonisstill40whenhetellsthestory.“Iwasalonethatnight,”hesays.“ThenIheardthemostbeautifulsoundinmylife.ItwasHuggons.”请在第____21____处填上正确答案。 A．before
B．while
C．since
D．until
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