—I joined the League ________ May, 2009. What about you?

—I've been a League member _______ three years. [ ]

A. in; for

B. on; in

C. on; for

D. in; since

数据结构是所研究对象的______的集合。这些对象是数据库的组成成分，一般分为两类，一类是与数据类型、内容有关的对象，另一类是与数据之间联系有关的对象。