题目3单选题
为teacher表的tel列创建一个唯一索引un_tel, 下列SQL语句写法是正确的是( )A. CREATE TABLE teacher ADD UNIQUE INDEX un_tel(tel)B. CREATE INDEX teacher ADD UNIQUE INDEX un_tel(tel)C. ALTER TABLE teacher ADD UNIQUE INDEX un_tel(tel)D. ALTER TABLE teacher MODIFY UNIQUE INDEX un_tel(tel)