Accordingtothepassage,agreatvarietyofinfluenzastr ainscanappearwhen______.
A.HandNspikesareproduced
B.animalandbirdvirusesarecombined
C.dissimilartypesvirusrecombine
D.twovirusesofthesametypearecontr acted
正确答案:C[解析]见第二段第二句话“Theresultofsuchrecombinationsisagreatvarietyofstr ainsconstr ainingdifferentHandNspikes.”。



