You are presented with the quiz "Plants may die without what?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "the looseness of soil increases the amount of oxygen in that soil" and "without oxygen in the soil, most plants die". So, what's the best answer to the question?

Select from: 1. nucleus of a cell; 2. some looseness in the soil; 3. wavelengths and photons; 4. moss; 5. single-celled organisms; 6. organic molecules; 7. snow; 8. rocks;
The answer is:
2.