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: Physiology
Which amongst the following is the gold standard for demonstrating hCG 
Select and explain. 
A. Radio immuno assay, B. Latex agglutination, C. Immunoflorescence, D. ELISA
A: Radio immuno assay
Answer: A: Radio immuno assay 
Explanation: None