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【单选题】
According to paragraph 1, why is it impossible to identify the time period for theatres in Greece?A.
There are too few sites that have been excavated and very little data collected about them.
B.
The archeologists from earlier periods were not careful, and many artifacts were broken.
C.
It is confusing because stones from early sites were used to build later structures.
D.
Because it is very difficult to date the concrete that was used in construction during early periods.
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【多选题】查询每个学生获得最高成绩的课程号,关于“select sno, cno, max(grade) mgrade from sc group by sno”语句,说法正确的有( )
A.
该语句是正确的
B.
该语句不对，因为cno没有包含在聚集函数或group by 子句中
C.
该语句可改为：（1）先建一个求每个学生最大成绩的视图，create view vmgrade as select sno,max(grade) mgrade from sc Group by sno; （2）再用这个学生的最高成绩找获得这个最高成绩的课程号 select sc.sno, cno from sc, vmgrade where sc.sno=vmgrade.sno and sc.grade=vmgrade.mgrade
D.
该语句可改为基于派生表的查询，以实现 select sno, cno from sc, (select sno, max(grade) from sc group by sno) as max_sc(sno, grade) where sc.sno=max_sc.sno and sc.grade>=max_sc.grade
【简答题】硅稳压管稳压电路正常工作时,稳压管工作在 状态。
【简答题】稳压管工作在 状态,发光管工作在 状态
【简答题】稳压电路中,稳压管工作在 状态。
【简答题】稳压管工作在 状态时,才会呈现稳压状态 。
【简答题】稳压管工作在 状态。
【多选题】一个好的公关活动主题,一般要考虑以下几个因素: ( )。
A.
表现公共关系目标
B.
不违背政府部门要求
C.
符合公众心理
D.
简明扼要,易记易传
E.
突出活动信息特性
F.
有利于塑造企业形象