This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous system, Subject name: Physiology
Frequency of Beta waves 
Select and explain. 
A. 5-10 Hz, B. 10-15 Hz, C. 13-30 Hz, D. None
C: 13-30 Hz
Answer: C: 13-30 Hz 
Explanation: When attention is focused on something, the alpha rhythm is replaced by an irregular 13-30 Hz low-voltage activity, the beta rhythm(Ref: Ganong's Review of Medical Physiology, Twenty-Third Edition)