Fact 1: Astove is used for cooking by generating heat. Fact 2: Warmth generated by staff and machinery heats the air, causing it to rise. Given the two facts above, a stove is used for cooking by heating the what?

Choose from: 1). air; 2). weather; 3). coal; 4). pan; 5). gas; 6). bed; 7). pets; 8). fire;
Answer:
1).