Concept:The primary source of water is the first-hand source from which all other sources are replenished. Rainwater is the original input to the water cycle.Explanation:Rainwater is considered the primary source because it directly adds new water to the Earth's ecosystem. All other sources—groundwater, lakes, and rivers—are secondary as they depend on rainwater for their supply. For example, groundwater comes from seepage of rainwater, lakes collect rainwater, and rivers are fed by rain or melting glaciers. Only rainwater continuously replenishes the lost water from these sources. Therefore, among the given options, rainwater is the original and main source of water on Earth.Answer:B. Rain water