The Sun is the ultimate source of energy for all living things.

• A. The Sun
• B. The Moon
• C. Earth's core
• D. Water

Answer: (A)

Energy for most life begins with sunlight. Plants capture light through photosynthesis and store that energy as chemical energy in sugars. When other organisms eat plants (or eat each other), that energy moves through the food chain and is ultimately used for growth, movement, and metabolism, with some of it lost as heat along the way. The Moon doesn’t generate usable energy for living systems; it only reflects sunlight. The Earth's core is hot, but geothermal energy isn’t the main energy source for surface ecosystems. Water is essential for life but doesn’t provide energy itself. So the Sun is the best answer.