This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Muscle, Subject name: Physiology
The ratio of actin : troponin : tropomyosin molecules in an thin filament is: 
Select and explain. 
A. 1:01:01, B. 1:03:05, C. 1:07:07, D. 7:01:01
D: 7:01:01
Answer: D: 7:01:01 
Explanation: A thin filament has 3 molecules - actin troponin tropomyosin in a ratio of 7: 1: 1.