员工表staff表结构如下
staffNo varchar2(10),
Email varchar2(50)
哪一个SQL语句能查询出没有E_mail地址的员工号（C ）
A. select staffno from Staff where Email = NULL
B. select staffno from Staff where Email <> NULL
C. select staffno from Staff where Email is null
D. select staffno from Staff where Email is not null