All venous blood samples were tested and no positive HIV antibodies were found. If the above determination is false, then the following _______ items must be true. ['To detect positive HIV antibodies, all venous blood samples must be tested.', 'Testing of all venous blood samples may not detect positive HIV antibodies.', 'Some venous blood samples have not been tested or positive HIV antibodies have been found.', 'Some venous blood samples have not been tested, but positive HIV antibodies have been found.']