Q: Fact 1: Afire requires oxygen to burn. Fact 2: They produce oxygen as a by-product of photosynthesis. Given the two facts above, a fire requires a by-product of what?

pick from the following. 1). sunlight 2). heat energy 3). electromagnetic energy 4). photosynthesis. 5). evaporation 6). heating liquids 7). Electrical energy 8). harmful substances
A:
4).