题目

—Jane, have you finished your’paper? —No,I        my sister with her homework all the morning. A.help              B.have been helping    C.helped           D.was helping   答案：B校园中仅松树就有好几个品种，这反应了生物多样性的哪一个层次？（　　）A.物种多样性B.生态系统多样性C.遗传多样性D.ABC都包括