You are presented with the quiz "What releases oxygen during the day but not during the night?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "plants release oxygen during the day but not during the night" and "plants include blue pine, hemlock, spruce and oak". So, what's the best answer to the question?

Pick your answer from:
1). elephants
2). dogs
3). hydrocarbons
4). blue pine
5). Conifers
6). vegetation
7). humans
8). Fuel cells
The answer is:
4).