What element is added to toothpaste and city water systems to tooth decay?

How does the dialogue between henry and his friends? “californians tale”

The rich natural resources made the sacramento valley a "garden of eden." permanent villages were established about 8,000 years ago. which choice combines these two sentences from paragraph two to achieve a coherent flow of ideas? a) the rich natural resources made the sacramento valley a "garden of eden" and permanent villages were established about 8,000 years ago. b) permanent villages were established about 8,000 years ago and the rich natural resources made the sacramento valley a "garden of eden." c) the rich natural resources made the sacramento valley a "garden of eden; " and so the permanent villages were established about 8,000 years ago. d) because of the rich natural resources which made the sacramento valley a ‘garden of eden,’ native americans established permanent villages there 8,000 years ago.

Which fits best in the empty box above?