Fact 1: Arefrigerator is used for keeping food fresh for a long period of time. Fact 2: Refrigerator is too cold. Given the two facts above, what keeps food fresh?

Available choices:
1). bacteria;
2). sodium;
3). lipids;
4). cold;
5). gas;
6). sugar;
7). cooled;
8). dust;
The answer is:
4).