The Davis Mountains in West Texas used to be taller than they are now. Which of the following conditions most likely caused the mountains to become shorter over time?
A: Rain and wind

During science class, students combined 1 kg of iron filings with 1 kg of sand to create a mixture. Which of the following methods would be best for separating the iron filings from the sand?
A: Drag a magnet through the mixture to attract the iron filings.

Which best explains the importance of nitrogen in the cycling of energy and matter?
A: Nitrogen increases protein production in plants.

Which of these provides most of the oxygen in Earth's atmosphere?
A:
ocean phytoplankton