Which of the following returns a NULL value? Select one or more: a. select ((10 + 20)50) + null from dual; b. select null/O from dual; O cselect concat(null,"test") as test from dual; Od select concat("this is a ", null,"test") from dual; The student and session table has an identical column named USI, which two(2) of the following statements joins these tables bases on common USA values? Select one or more select * from student inner join session on studentusi = session usi Obselect from student natural join session on studentusi=sessionusi select * from student natural join session Od select from student inner join session using ust