Выражение встречается в следующем контексте:
Suppose, we study the demand for tube travel. A rise in bus fares or petrol prices would increase the quantity of tube travel demanded at each possible price. In everyday language, we think of buses and private cars as substitutes for the tube. Loosely speaking, this means that a journey may be made by bus or car instead of by tube. More precisely, an increase in the price of buses or cars will lead to an increase in the demand for tube travel

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