什么是账单周期？
参考答案：
账单周期是指上一账单日次日到本账单日的时间周期。
在生物的分类等级单位中，______ 是最基本分类单位，______ 生物的亲缘关系最密切的．
_____ is my father. _____ name is Dave Smith. [ ]
A. He; He's
B. His; He
C. He; His
D. He's; His
