多选题：在Oracle数据库中，通过create table t as select * from dba_objects;语句创建T表，同时通过create index idx_t_object_id on t(object_id) 创建索引，T表的object_id可以为NULL。请问以下哪些SQL语句会利用到索引返回记录？BCDEF A、select count(*) from t; B、select avg(object_id) from t ； C、select count(object_id) from t; D、select avg(object_id) from t where object_id is not null; E、select count(object_id) from t where object_id is not null; F、select count(*) from t where object_id is not null;