Fact 1: Friction causes an object to lose energy. Fact 2: Friction occurs when surfaces rub together. Given the two facts above, what causes an object to lose energy?

Select from the following. (1). high temperature. (2). electron transport. (3). degrees celcius. (4). surfaces rubbing together. (5). heat is produced. (6). degrees Celsius. (7). depress neuronal excitability. (8). a tree falling.
The answer is:
(4).