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: Dental
1 part per million fluoride is: 
Select and explain. 
A. 1 mg in 10 L of water, B. 1 mg in 1 L of water, C. 10 mg in 1 L of water, D. 100 mg in 10 L of water
B: 1 mg in 1 L of water
Answer: B: 1 mg in 1 L of water 
Explanation: ppm (or ppmm) is used to measure the concentration of a contaminant in soils and sediments. In that case, 1 ppm equals 1 mg of substance per liter of solid (mg/liter).