What does it mean to have a 'heart of steel'?

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.