在Oracle中，有一个教师表teacher的结构如下：
ID NUMBER(5),NAME VARCHAR2(25),EMAIL VARCHAR2(50)
下面（ ）语句显示没有Email地址的教师姓名。
Aselect name from teacher where email=null;
Bselect name from teacher where emailnull;
Cselect name from teacher where email is null;
Dselect name from teacher where email is not null;