现有表Employee，字段：id（int），firstname（varchar）,lastname（varchar）;以下sql语句正确是哪三项？
A.selectfirstname+'.'+lastnameas'name'fromemployee
B.selectfirstname+'.'+lastname='name'fromemployee
C.select'name'=firstname+'.'+lastnamefromemployee
D.selectfirstname,lastnamefromemployee