Excess nutrients in water drive eutrophication, where algal blooms grow rapidly and then decompose, using up the dissolved oxygen in the water and creating hypoxic conditions. For the Gulf of Mexico, the main nutrient culprit is nitrogen carried by the Mississippi River from agricultural runoff. This nitrogen, in the form of nitrates and related compounds, fuels large phytoplankton blooms that, when they die and are decomposed by microbes, consume a lot of oxygen and leave a bottom-wide dead zone. Phosphorus can contribute, but the extent and timing of the Gulf dead zone are most closely tied to nitrogen loading. Potassium isn’t a primary driver of algal growth in this context, and ammonia is just a form of nitrogen, whereas the element at issue is nitrogen itself.

Intermittency is the uneven production of renewable energy, since solar and wind only generate power when conditions allow. Energy storage directly tackles this by capturing excess electricity when generation is high and releasing it during periods of low or no generation, helping to keep the grid stable and meet demand consistently. This smoothing effect is essential for renewables to replace fossil fuels reliably, as storage can be in batteries, pumped hydro, or other forms that match supply with demand across short-term and even longer-term gaps. The other options don’t address the timing of supply. Drilling for oil creates another energy source but doesn’t fix when renewables produce power or help balance the grid. Reducing planning would undermine system reliability, not improve it. Removing subsidies affects economics, not the immediate ability to balance supply and demand with stored energy.

Measuring environmental sustainability needs a broad view that includes how much people consume, how waste is handled, and the health of natural resources. Per capita resource use shows the average demand each person places on resources, signaling whether consumption is staying within the long-term supply. Recycling rates reflect how effectively a community reduces waste and recovers materials, indicating progress toward a more resource-efficient system. Groundwater levels reveal the status of an essential resource that can be stressed by over-extraction or contamination, highlighting potential future shortages. Because these indicators capture different parts of the system—demand, waste management, and resource availability—using them together gives a fuller, more reliable assessment than any single measure. While each one on its own provides useful insight, they collectively offer a robust picture of environmental sustainability.

Renewable resources replenish naturally on human timescales, while non-renewable resources are finite. This distinction is central to sustainability because it shapes how we think about long-term availability and how we plan, manage, and transition resources. If a resource is renewable, the issue becomes using it at a rate that does not exceed its replenishment and dealing with challenges like intermittency and ecological impact. If a resource is non-renewable, the concern is depletion and the need for efficiency, substitution, or a shift to alternatives as reserves fall. This idea underpins case studies that compare future supply, environmental costs, and policy choices. It’s also important to recognize that renewables are not automatically abundant and do not remove all pollution, and that some non-renewables can have lower emissions or costs in certain contexts.

The main idea here is identifying pollutants that are most prominent in urban air and linking them to common health effects residents may experience. Fine particulate matter (PM2.5), nitrogen oxides (NOx), and sulfur dioxide (SO2) are classic urban pollutants from vehicles, industry, and combustion. PM2.5 particles are so small they can penetrate deep into the lungs and enter the bloodstream, contributing to respiratory problems and cardiovascular disease. NOx gases irritate airways and can worsen asthma and other respiratory conditions, and they also help form ground-level ozone, which adds to health risks. SO2 can cause bronchoconstriction and throat irritation, and exposure can aggravate asthma and other lung issues. Pairing these pollutants with respiratory problems, cardiovascular disease, and aggravated asthma reflects the well-established health impacts seen in urban populations. The other options don’t fit as well. A mix that includes CO2 and CH4 emphasizes climate change rather than direct, everyday urban health effects, and while ozone is a pollutant, the trio isn’t representative of the main urban pollutants. A group like Lead, Mercury, Cadmium includes toxic metals, but the listed health effects (skin irritation and hair loss) don’t accurately reflect their typical health impacts, and these metals aren’t the most representative trio for urban air quality case studies. Finally, water vapor, nitrogen, and argon are largely inert in everyday exposure and don’t convey meaningful health impacts in this context.