Natural Science CLEP Prep 2025 – 400 Free Practice Questions to Pass the Exam

Most ecosystems rely on sunlight as their primary source of energy. Carbon dioxide is necessary for photosynthesis, but it is not the primary source of energy. While oxygen is necessary for life, it is not the primary source of energy. And while water is necessary for life, it is not the primary source of energy in most ecosystems. Sunlight provides the energy needed for photosynthesis, which is the process by which plants and other organisms convert light energy into chemical energy, making it the primary source of energy for most ecosystems.