I think the answer is ice
Nitrogen released from dead matter goes into soil, helping/feeding the plants, who in turn give us oxygen
The correct answer is secondary sexual characteristic.
These are the characteristics that start developing in humans during puberty. Primary sex characteristics are obviously reproductive organs, but secondary characteristics refer to pubic hair, enlarged breasts with females, and facial hair/Adam's apple with males.
The correct answer to the question above is geothermal energy. Some say that the benefit of a volcanic activity, which could be the geothermal energy that could be produced by it, outweighs its hazards in the long-term process.