This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CVS Embryology, Subject name: Anatomy
Closure of the foramen primum results from fusion of which of the following structures? 
Select and explain. 
A. Septum secundum and the fused atrioventricular cushions, B. Septum secundum and the septum primum, C. Septum primum and the fused atrioventricular cushions, D. Septum primum and the septum spurium
C: Septum primum and the fused atrioventricular cushions
Answer: C: Septum primum and the fused atrioventricular cushions 
Explanation: C. The foramen primum forms between the free edge of the septum primum and the atrioventricular (AV) cushions. It is closed when the septum primum fuses with the AV cushions.