Freshwater is considered a limited resource primarily due to the fact that a large majority of Earth's water is saltwater (option A). Here’s a bit more detail on each option:
a. Most water covering Earth is salt water: This is true; approximately 97% of the Earth's water is saltwater found in oceans. Only about 3% of the Earth's water is freshwater, and much of that is trapped in glaciers and ice caps, making the accessible freshwater supply even smaller.
b. Groundwater is too difficult to access: While groundwater is a significant source of freshwater, accessing it can be challenging in some areas due to geological formations or technology limitations. However, this is not the main reason freshwater is limited.
c. The supply of water on Earth decreases over time: This is not accurate in the broader sense because the total amount of water on Earth doesn't decrease. However, local water supplies can become depleted due to overuse and pollution.
d. Too much water is wasted on irrigation: This is a valid concern, as inefficient irrigation practices can lead to water wastage. However, it is a contributing factor rather than the primary reason for freshwater scarcity.
In summary, while there are various factors affecting freshwater availability, the most fundamental reason is that a vast majority of the Earth's water is saltwater (option A).