Which of the following statements about acid precipitation is not true?

a) an increase in the use of buses and trains decreases the effects of acid precipitation.

b) acid precipitation forms when sulfur dioxide and nitrogen oxides combine with atmospheric moisture to create sulfuric acid and nitric acid.

c) a decrease in the use of cars would mean fewer nitrogen oxides would pour into the air and less acid precipitation would form.

d) most nitrogen oxides that cause acid precipitation come from factories.