Choices:
(1). The atmosphere
(2). kinetic energy
(3). smoke
(4). crop yields
(5). nutrients
(6). Temperature
(7). fires
(8). smog

Fact 1: Air pollution can cause the pH of soil to decrease. Fact 2: Research has shown that soil pH less than 6.0 can decrease crop yields. Given the two facts above, air pollution can cause a decrease in what?
Answer:
(4).