以下关于Oracle外连接的写法，哪些是正确的？BD A select * from emp,dept2 where emp.deptno=dept2.deptno(+) and dept2.deptno=20 ; B select * from emp,dept2 where emp.deptno=dept2.deptno(+) and dept2.deptno(+)=20 ; C select * from emp left join dept2 on emp.deptno=dept2.deptno where dept2.deptno=20 ; D select * from emp left join dept2 using(deptno) ;