抗震设计时，钢筋混凝土构造柱、芯柱、圈梁等混凝土强度等级不应低于（）

A.C20

B.C25

C.30

D.C40

假设供应商S和供应情况SPJ的关系模式分别为S(Sno，Sname，Status，City)和SPJ (Sno，Pno，Jno，Qty)。SQL语句 (25) 不能正确地查询出“零件号Pno等于’P3’的供应商名Sname"，而 (26) 能正确查询的关系代数表达式。

A．SELECT DISTINCT Sname
FROM S
WHERERE 0 ＜ (SELECT COUNT( * )FROM SPJ WHERE S. Sno = SPJ. Sno
AND SPJ. Pno = ’P3’)

B．SELECT DISTINCT Sname
FROM S
WHERE EXISTS(SELECT * FROM SPJ WHERE S. Sno=SPJ. Sno
AND SPJ. Pno= ’P3’)

C．SELECT Sname
FROM S
WHERE EXISTS( SELECT * FROM SPJ WHERE S. Sno = SPJ. Sno AND SPJ. Pno = ’P3’)

D．SELECT Sname
FROM S, SPJ
WHERE S. Sno = SPJ. Sno AND SPJ. Pno = ’P3’
GROUP BY Sname