某企业每月固定成本2000元,单20元,计划销售产品500件,欲实现目标利润1000,其单位变动成本应为()
A.12
B.13
C.14
D.15
参考答案:C
I have been trying to advise them not to act that way, but my advice had no _________on them.
A.impression
B.advantage
C.influence
D.affect
若关系中的某一属性组的值能唯一地标志一个元组,则称该属性组为( )。
A.主键
B.候选键
C.主属性
D.外部键