积分result表中有A B C D四列，要求：1）当A列值大于等于B列时，选择A列否则选择B列2）当C列值大于等于D列时，选择C列否则选择D列用SQL语句实现正确的是：（      ） A: select(whenA>=BthenAelseB)MAX_AB,(whenC>=DthenCelseD)MAX_CDfromresult B: select(casewhenA>=BthenAelseB)MAX_AB,(casewhenC>=DthenCelseD)MAX_CDfromresult C: select(casewhenA>=BthenAelseBend)MAX_AB,(casewhenC>=DthenCelseDend)MAX_CDfromresult D: selectcasewhenA>=BthenAelseBendMAX_AB,casewhenC>=DthenCelseDendMAX_CDfromresult