This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Drug Abuse, Subject name: Forensic Medicine
This instrument is based on the principle of 
Select and explain. 
A. Henry's law, B. Widmarck's law, C. Osmosis, D. Neutron activation analysis
A: Henry's law
Answer: A: Henry's law 
Explanation: This is based on Henry's law which states that when a volatile chemical (ethanol) is dissolved in a liquid (blood) and is brought to equilibrium with air (alveolar breath), there is a fixed ratio between the concentration of the volatile compound (ethanol) in air (alveolar breath) and its concentration in the liquid (blood), and the ratio is constant at a given temperature. Correlation between the breath and blood level, and the ratio is generally 2100 : 1.