— There's ______ time. You don't need to hurry, Bob. 

— But I want to see her as early as possible. [ ]

A. little

B. many

C. a lot

D. plenty of

(20)～(23)题基于“学生-选课-课程”数据库中的三个关系：
S(S#，SNAME，SEX，AGE)， SC(S#，C#，GRADE)， C(C#，CNAME，TEACHER)

为了提高查询速度，对SC表(关系)创建惟一索引，应该创建在哪个(组)属性上( )

A．S#，C#

B．S#

C．C#

D．GRADE