Various use of CASE statement

Q. Which query return different value?

1. SELECT CASE 0 WHEN 0 THEN 'A' ELSE 'B' END

FROM DUAL;

2. SELECT CASE WHEN 0 = 0 THEN 'A' ELSE 'B' END

FROM DUAL;

3. SELECT CASE NULL WHEN NULL THEN 'A' ELSE 'B' END

FROM DUAL;

4. SELECT CASE WHEN NULL IS NULL THEN 'A' ELSE 'B' END

FROM DUAL;