Final answer:
The necessary amount of water decreases faster when there is excessive water loss due to health conditions such as diseases or environmental factors like severe water shortages, which can lead to rapid dehydration and scarcity.
Step-by-step explanation:
The necessary amount of water tends to decrease faster when there is excessive water loss due to various factors that outweigh water intake, leading to a condition known as dehydration. For example, during plant transpiration, the plant may need less water as the temperature increases, causing transpiration to slow down and thus decrease water uptake by the roots (option b). However, this process does not typically result in a rapid decrease in necessary water amounts compared to conditions that involve excessive water loss, such as diseases (option c.1). Water loss in diseases like diarrhea, vomiting, kidney disease, and fever can be rapid and sometimes fatal, especially in vulnerable populations like infants (option C.1).
Water shortages also play a critical role in water scarcity. Humans contribute to the occurrence of droughts by activities such as deforestation, which reduces the moisture added to the air and makes the droughts more common. In areas with severe water shortages, the necessary amount of water needed for survival can decrease rapidly as the water becomes unavailable (option Water Shortages).