在汽车维护中，（）维护属于强制维护。
A.日常
B.一级
C.二级
参考答案：C
试述主键和外部键的完整性约束条件，并说明其原因。
It is a market which sales value might be more than 10 billion yuan.()
A. a market with a sales value that might be
B. a market which might be sales value
C. a market with sale value might be
D. market with sales might be a value
