The statement that lymph returns to the bloodstream primarily through the thoracic duct and right lymphatic duct is a matter of established anatomical fact and direct observation, requiring no citation.
The claim is a foundational, well-established physiological fact taught in basic human anatomy regarding lymphatic drainage into the venous system. According to the instructions, statements that are trivially true by direct everyday observation or textbook definition are categorized as COMMON_KNOWLEDGE, requiring no papers or citations.