Fact 1: Air pollution can cause the pH of soil to decrease. Fact 2: Some soils are more sensitive than others to the acid dropped by air pollution. Given the two facts above, some soils are more sensitive than others to a lowered what?

Choose from: i. p53 ii. HPV iii. CO2 iv. wolf population v. light level vi. pH vii. sea level viii. dam
Answer:
vi.