One issue with monitoring freshwater sources is that detecting pollution in underground water can take years. Option A is the right option.
What is underground water?
Groundwater is the water that exists beneath the Earth's surface in the pore spaces of rocks and soils, as well as in the fractures of rock formations.
Groundwater makes up about a third of all readily available freshwater on the planet. When a unit of rock or an unconsolidated deposit may supply an useable amount of water, it is called an aquifer.
Its pollution cannot be detected in some minimum time, it takes long time to detect the pollution in underground water. This problem arises with monitoring freshwater supplies.
Therefore, option A is correct.
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