What is associated with: about 7000 years ago mexican indians improved this crop notably the size of its ears?

Based on the clue 'about 70 feet high each one of these obelisks is i...', what is the answer? Based on the clue 'about 70 of chinas rice crop is grown in this rive...', what is the answer? Based on the clue 'about 70 of the iron we absorb ends up in this pro...', what is the answer? Based on the clue 'about 70 of this pungent gas produced is converted...', what is the answer? What is associated with: about 70000 now live in this planned new town founded in 1967 between baltimore dc? Based on the clue 'about 75 of all hawaiians live on this island', what is the answer? Based on the clue 'about 75 of all us coins are minted in this capita...', what is the answer? Based on the clue 'about 75 of the corn sold by farmers in the us is ...', what is the answer?