studentName字段有索引，下列SQL中索引会失效的是？

studentName字段有索引，下列SQL中索引会失效的是？

A.SELECT * FROM student WHERE studentName = 123456;

B.SELECT * FROM student WHERE studentName NOT IN ("李白""韩信""裴擒虎");

C.SELECT * FROM student WHERE studentName = '123456';

D.SELECT * FROM student WHERE studentName !='李白';

正确答案是ABD