use dynamic sql as user A:

set verify off
set term off
set heading off
set feedback off
set pause off
set trimspool on
set linesize 1024
set pagesize 0
clear breaks
clear columns

spool grant.sql

select 'grant select, insert, update, delete on '||
table_name || ' to USER_A;'
from user_tables;

spool off
@grant.sql