Final answer:
The thoracic duct and the right lymphatic duct empty lymph into the subclavian veins, with the right lymphatic duct draining into the right subclavian vein and the thoracic duct into the left subclavian vein.
Step-by-step explanation:
The thoracic duct and the right lymphatic duct are both part of the lymphatic system, which helps in the maintenance of fluid balance and the defense against infections. The right lymphatic duct drains lymph fluid from the upper right side of the body, including the right sides of the head, thorax, and right upper limb, directly into the right subclavian vein. The thoracic duct, which is much larger, begins beneath the diaphragm in the cisterna chyli and receives lymph from the lower abdomen, pelvis, and lower limbs, before draining into the left subclavian vein.
Therefore, considering the drainage sites for these two ducts, the correct answer to the question 'The thoracic duct and the right lymphatic duct empty lymph into which vessel?' is B) Subclavian veins.