This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
All the following openings in the right atrium are guarded by valve except - 
Select and explain. 
A. Superior vena cava, B. Inferior vena cava, C. Coronary sinus, D. Atrioventricular opening
A: Superior vena cava
Answer: A: Superior vena cava 
Explanation: 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