假设供应商S和供应情况SPJ的关系模式分别为S(Sno，Snaale，Status，City)和SPJ(Sno，Pno，Jno，Qty)。SQL。语句(19)不能正确地查询出“零件号Pno等于‘P3’的供应商名Snam”，而(20)能正确查询的关系代数表达式。
A．SELECT DISTINCTSname
B．SELECT DISTm CTSname FROMS FROMS WHERE EXISTS WHERE 0＜(SELECT*(SELECTCOUNT(*)FROM SPJ FROM SPJ WHERE S．Sno=SPJ．SnO WHERE S．Sno；SPJ．Sno AND SPJ．Pno；
C．SELECT Shame
D．SELECT Sname FROMS FROMS，SPJ WHERE EXISTS WHERES.Sno=SPJ.Sno(SELECT* ANDSPJ．Pno='P3')FROM SPJ GROUP BYSname WHERE S. Sno=SPJ. Sno
