There is a lot of evidence that fluoride reduces cavities but not all communities add it to their drinking water. A dentist, who expects to replicate this earlier work, classifies randomly selected communities in his state as (1) adding fluoride to their drinking water or (2) not adding fluoride to their drinking water. He then goes to high schools in these communities, inspects the mouths of all high school seniors, and calculates, for each community, the percentage of these students with cavities. He will compare the means for these values between the two types of communities.

(a) Write out H₀ and H₁ and

(b) Explain what they mean.