The question tests the historical origin of fuel cells and when the first practical design was created. In 1839, Sir William Grove built the first fuel cell, known as Grove’s gas voltaic cell. He used hydrogen and oxygen separated by an electrolyte to generate electricity directly from a chemical reaction, producing water as a byproduct. This established the fundamental idea of converting chemical energy from fuel directly into electrical energy without combustion, making 1839 the landmark year for the birth of fuel-cell technology. The other dates don’t fit the historical timeline: 1639 and 1739 are far too early for this technology, and 1939 refers to a later stage in the field, not its inception.

Delivering water where plants need it with minimal losses is the key to water-use efficiency, and drip irrigation does this best. By releasing water slowly and directly into the root zones through emitters placed along plant beds or around individual plants, it avoids spraying water into the air and onto dry surfaces. That means far less evaporation, wind drift, and runoff, plus less watering of weed-prone areas. Drip systems are also highly adaptable: they can be installed underground or laid on the surface, and you can create zones so plants with different water needs are irrigated appropriately. They pair well with mulch, which further reduces surface evaporation, and, with proper filtration and pressure regulation, keep emitters from clogging and ensure uniform delivery. While sprinklers and surface irrigation have roles in certain situations, they generally fall short on efficiency due to higher evaporation, wind losses, or uneven infiltration. Fogging is specific to greenhouse environments and isn’t a typical landscape irrigation method.

The main idea here is understanding what each process does with different types of waste. Recycling takes materials such as paper, glass, plastics, and metals and reprocesses them into new products, effectively turning used items back into raw materials for manufacturing. Composting, on the other hand, uses microbes to biologically break down organic waste—like food scraps and yard waste—into a nutrient-rich soil amendment that improves soil health. That’s why the correct statement fits: recycling reprocesses materials into new products, while composting transforms organic waste into soil amendments. The other choices misstate what each process does—recycling and composting don’t both turn waste into energy, composting isn’t for plastics, and composting doesn’t yield metal pellets while recycling doesn’t create soil.

The most expensive audit is the one that goes deepest into how energy is actually used across systems, with rigorous diagnostics and engineering analysis. A technical/engineering audit involves detailed on-site measurements of equipment performance, energy balances, and system integration, often requiring specialized instruments, multiple engineers, and longer field work. It may also involve engineering calculations or simulations to quantify savings and develop capital-justified recommendations, which drives up cost. By comparison, a lighting audit is narrower and quicker, focusing on lighting systems with simpler measurements; a preliminary audit is high-level and low-cost by design; and a process improvement audit, while potentially data-intensive, usually doesn’t demand the same level of engineering testing or system-wide modeling as a full technical audit.