用所给词的适当形式填空。

     I'm Carol. Do you know how I study English? Well, vocabulary is the 1.________ (important) thing for me to study.  I always listen to the English program on the radio. When I hear some new words, I try 2.        (keep) them in my mind. Then after one or 3. ______(twice) days, I still remember some of them. I don't learn English just from English lessons. I learn it in 4.______ (difference) ways. Of course, I read some 5.        (interest) stories in English. I also speak a lot. To speak to myself is a good way. Many people don't like to do that. But I like doing that.

若关系中的某一属性组的值能唯一地标识一个元组。则称该属性组为( )。

A．主键
B．候选键
C．主属性
D．外部键