What special structure is on the posterior wall of the right atrium

The coronary sinus
The coronary sinus is the special structure located on the posterior wall of the right atrium. It is an important structure as it provides information on both left atrial and left ventricular activity without requiring a trans-septal approach (a technique for patients who have valvular disease). The coronary sinus serves as the primary collector of cardiac venous blood and it facilitates the movement of blood from the myocardium (a thick layer of muscle within the heart) into the right atrium. The coronary sinus also plays important roles when performing cardiac surgery and in other heart procedures.

Idk i think the answer choice is number 1