Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: As the human population increases, more fossil fuels are being used. The burning of fossil fuels produces greenhouse gases. How might humans reduce greenhouse gases?
Use renewable resources to generate power.
Use renewable energy sources, increase energy efficiency.