将未患有所要研究疾病的人群分为暴露组和非暴露组， 随访观察两组人群某事件发生率的差别，判断某因素与事件的关系的研究方法属于()
C
暂无解析
Thebackgardenofourhousecontainsalawn，____verypleasanttositoninsummer.
John'sscoreonthetestisthehighestintheclass;（）helastnight.
Modern____perhapscausesmoreproblemsthanitsolves.
Californiaisalandofvarietyandcontrast.Almosteverytypeofphysicallandfeature，sortofarcticicefieldsandtropicaljunglescanbefoundwithinitsborders.Sharplycontrastingtypesoflandoftenlieveryclosetooneanother.PeoplelivinginBakersfield，forinstance，canvisitthePacificOceanandthecoastalplain，thefertileSanJoaquinValley，thearidMojaveDesert，andthehighSierraNevada，allwithinaradiusofabout100miles.Inotherareasitispossibletogosnowskiinginthemorningandsurfingintheeveningofthesameday，withouthavingtotravellongdistance.ContrastaboundsinCalifornia.ThehighestpointintheUnitedStates(outsideAlaska)isinCalifornia，andsoisthelowestpoint(includingAlaska)。MountWhitney，14，494feetabovesealevel，isseparatedfromDeathValley，282feetbelowsealevel，byadistanceofonly100miles.Thetwoareashaveadifferenceinaltitudeofalmostthreemiles.Californiahasdeep，clearmountainlakeslikeLakeTahoe，thedeepestinthecountry，butitalsohasshallow，saltydesertlakes.IthasLakeTulainyo，12，020feetabovesealevel，andthelowestlakeinthecountry，theSaltonSea，236feetbelowsealevel.Someofitslakes，likeOwensLakeinDeathValley，arenotlakesatall：theyaredrieduplakebeds.Inadditiontomountains，lakes，valleys，deserts，andplateaus，CaliforniahasitsPacificcoastline，stretchinglongerthanthecoastlinesofOregonandWashingtoncombined.
1.WhichofthefollowingisthelowestpointintheUnitedStates?
A.LakeTulainyoB.MojavedesertC.DeathValleyD.TheSaltonSea
2.WhereisthehighestpointintheUnitedStateslocated?
A.LakeTahoe.
B.SierraNevada.C.MountWhitney.
D.Alaska.
3.HowfarawayisDeathValleyfromMountWhitney?
A.About3miles.
D.14，494feet.
4.WhichofthefollowingisNOTmentionedinthepassageasbeingwithinaradiusofabout100milesofBakersfield?
A.ThePacificOcean.
B.SanJoaquinValley.
C.MojaveDesert.
D.OregonandWashington.
5.WhichstatementbestdemonstratesthatCaliforniaisalandofvarietyandcontrast?
A.ThehighestlakeinCaliforniaisLakeTulainyo.
B.ItispossibletogosurfingandsnowskiinginsomepartsofCaliforniawithouthavingtotravellongdistance.
C.SierraNevada，SanJoaquinValley，MojaveDesertandthePacificOceanallliewithinaradiusofabout100miles.
D.OwensLake，inDeathValley，isnotreallyalakeatall.
Nowherebutintheremotestregionofthecountry____findaplacetosettledown.