题目内容：
根据以下材料回答题 InSeptember,inBritain,youmayseealotofbirds36onroofsandtelegraphwires．Thesebirdsareswallows．Theyare37togetherbecause,verysoon,theywillbeflying38tomuchwarmerlands,wheretheywillfind39smallflyinginsectsonwhichthey40Therearenosuchinsects41inBritainduringthewinter,asitis42．coldforthem.
Theswallowssettle,flyoff,swoop,and43again．Thistheydomanytimes,fortheyaremakingshort44flightsinordertobefitforthelongjourney45them.
46ofthesemigratingbirdsleaveBritainintheautumn．Theyfly47forhundredsofmiles48theyreachthewarmlandsofAfrica．Butnotallthebirdsgetthere,formanyofthemperishinthestormyweathertheymeetwith49.
Inthespringofthefollowingyearthey50thelongandtiringjourneybacktoBritain.
Theyreturntotheidenticalbarnortreeinthe51districtwhichtheyhadleftthe52autumn.
Howdothesebirdsfindtheir53thereandbackoversuchvastdistances?Nobodyknowsex-actly54,butithassomethingtodo55windsandaircurrents.
第36题应选 A．beingperched
B．perching
C．beingperching
D．beperched
参考答案：
答案解析：