20. Jet propulsion involves ______ of air and fuel, which forms a powerful exhaust. A) a mixture is ignited B) to ignite a mixture C) a mixture of igniting D) the ignition of a mixture

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.