以下哪些查询的结果是 2019-06-30 ？（ABCD） A、select add_months(date'2019-07-31',-1) from dual; B、select add_months(date'2019-07-30',-1) from dual; C、select add_months(date'2019-05-31',1) from dual; D、select add_months(date'2019-05-30',1) from dual;