SQL & PL/SQL

Hi all;

U1> grant references on u1.emp to scott;

Grant succeeded.

SCOTT>alter table u1.emp modify(emp_id number constraint  emp_c1_pk_eid  primary key);

alter table u1.emp modify(emp_id number constraint emp_c1_pk_eid primary key)

*

ERROR at line 1:

ORA-01031: insufficient privileges

To create  primary key or foreign key into  other's object , what privilege should i need ?

Thanks