(1). gravity.
(2). mammals.
(3). animals.
(4). trees.
(5). humans.
(6). density.
(7). volume.
(8). potatoes.

Fact 1: All life relies on a relatively narrow range of pH, or acidity. Fact 2: Human life is dependent life. Given the two facts above, what relies on a relatively narrow range of acidity?
Answer:
(5).