All the following openings in the right atrium are guarded by valve except -
Valves on different openings to Right atrium
-        Inferior vena cava- Rudimentary semilunar valve- Eustachian valve
-        Coronary sinus- Incomplete semicircular valve- Thebesian valve
-        AV opening- Tricuspid valve
Superior vena cava
Inferior vena cava
Coronary sinus
Atrioventricular opening