What Is the Optimal Concentration of Fluoride in a Community

What is the optimal concentration of fluoride in a community water supply?

A) 0) 12 ppm mg/L. 0.12mg/L or 0.12ppm (measured in mg/L OR parts per million {ppm}) One format should be used, not both in the same answer.
B) 0) 7 to 1.2 ppm mg/L. 0.7 to 1.2mg/L or 0.7ppm to 1.2ppm
C) 0) 7 ppm mg/L. 0.7mg/L or 0.7ppm
D) 0) 2 to 1.7 ppm mg/L. 0.2 to 1.7mg/L or 0.2ppm to 1.7ppm