Five definitions: fossil fuels, smog, organic farming, the grassland ecosystem, food web

Five of the six terms are defined, as the question allows: (i), (iii), (iv), (v) and (vi).

(i) Fossil fuels

Fossil fuels are coal, oil and natural gas: hydrocarbon fuels formed from the buried, fossilised plant life of ancient prehistoric forests. They are non-renewable because the stock cannot be replaced once used, and drilling, refining, transporting and burning them pollute air and water. Petrol and diesel in vehicles and coal in power stations are everyday examples.

(iii) Smog

Smog (smoke + fog) is a polluted haze that forms when smoke and gases such as sulphur oxides mix with fog in cold, still air trapped near the ground. It lowers visibility and causes coughing, sore throat and lung disorders; the London smog, in which the city's fog turned black into a pea-soup smog, is the classic case. Delhi's winter smog is a familiar Indian example.

(iv) Organic farming

Organic farming is growing crops without synthetic fertilisers and pesticides, keeping the soil fertile through added organic matter such as compost and manure, crop rotation and natural pest control. Organic matter keeps soil from compacting, and crop rotation raises fertility, reduces erosion and controls pests. Critics say it cannot feed today's population and pays only in some conditions; supporters reply that it is worth it once the hidden costs of erosion and pollution are counted.

(v) The grassland ecosystem

A grassland ecosystem is an area covered mainly by grasses and herbs, with few trees, found where rainfall is low or the soil is thin and poor. The grass grows in the monsoon, dries in summer and grows back from roots and seeds the next year, giving it a strongly seasonal look; its grasses, insects, birds and mammals are adapted to wide open land. The Himalayan pastures and the tall grasslands of the Terai are Indian examples.

(vi) Food web

A food web is the network formed when many food chains of an ecosystem are interconnected, because most animals eat more than one kind of food and some, such as crows, are omnivores. Grazing and detritus food chains are also linked through it. In a pond, a frog may eat insects and be eaten by both snakes and herons, so its food links form a web, not a single chain.