13. Every year Canadian ______ about 75 percent of their exports to the United States A) businesses that sell B) selling businesses C) businesses sell D) that sell to businesses

16. Even at low levels, ______ A) the nervous system has produced detrimental effects by lead. B) lead’s detrimental effects are producing the nervous system. C) lead produces detrimental effects on the nervous system. D) the detrimental effects produced by lead on the nervous system.