PassageTwo
Questions26to30arebasedonthefollowingpassage.
Scientistshaveestablishedthatinfluenzavirusestakenfrommancancausethediseaseinanimals.Inaddition，mancancatchthediseasefromanimals.Infact，agreatnumberofwildbirdsseemtocarrytheviruswithoutshowinganyevidenceofillness.Somescientistsconcludethatalargefamilyofinfluenzavirusesmayhaveevolvedinthebirdkingdom，agroupthathasbeenontheearth100millionyearsandisabletocarrytheviruswithoutcontr actingthedisease.Theseareevenconvincingevidencetoshowthatvirusstr ainsaretr ansmittedfromplacetoplaceandfromcontinenttocontinentbymigratingbirds.
Itisknownthattwoinfluenzavirusescanrecombinewhenbotharepresentinananimalatthesametime.Theresultofsuchrecombinationsisagreatvarietyofstr ainsconstr ainingdifferentHandNspikes.Thisraisesthepossibilitythatahumaninfluenzaviruscanrecombinewithaninfluenzavirusform.aloweranimaltoproduceanentirelynewspike.
Researchisunderwaytodetermineifthatisthewaythatmajornewstr ainscomeintobeing.Anotherpossibilityisthattwoanimalinfluenzastr ainsmayrecombineinapig，forexample，toproduceanewstr ainwhichistr ansmittedtoman.
Accordingtothepassage，scientistshavediscoveredthatinfluenzaviruses______.
A.causeillhealthinwildbirds
B.donotalwayscausesymptomsinbirds
C.arerarelypresentinwildbirds
D.changewhentr ansferredfromanimalstoman
正确答案：B[解析]见本文第三句话。
