This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Biostatistics, Subject name: Social & Preventive Medicine
[?](pq/n) is 
Select and explain. 
A. Standard error of propoions, B. Standard error of means, C. Standard error of propoions difference, D. Standard error of means difference
A: Standard error of propoions
Answer: A: Standard error of propoions 
Explanation: Ref:Park's Textbook of Preventive and Social Medicine 25th Ed