<P> There are different degrees of substitutability . For example, a car and a bicycle may substitute to some extent: if the price of motor fuel increases considerably, one may expect that some people will switch to bicycles . </P> <P> In economics, one way that two or more goods can be classified is by examining the relationship of the demand schedules when the price of one good changes . This relationship between demand schedules leads to classification of goods as either substitutes or complements . Substitute goods are goods which, as a result of changed conditions, may replace each other in use (or consumption). A substitute good, in contrast to a complementary good, is a good with a positive cross elasticity of demand . This means a good's demand is increased when the price of another good is increased; both in the same direction . Conversely, the demand for a good is decreased when the price of another good is decreased . If goods A and B are substitutes, an increase in the price of A will result in a leftward movement along the demand curve of A and cause the demand curve for B to shift out . A decrease in the price of A will result in a rightward movement along the demand curve of A and cause the demand curve for B to shift in . </P> <P> Examples of substitute goods include margarine and butter, tea and coffee, beer and wine . Substitute goods not only occur on the consumer side of the market but also the producer side . Substitutable producer goods would include: petroleum and natural gas (used for heating or electricity). The degree to which a good has a perfect substitute depends on how specifically the good is defined . Take for example, the demand for Rice Krispies cereal, which is a very narrowly defined good as compared to the demand for cereal generally . The fact that one good is substitutable for another has immediate economic consequences: insofar as one good can be substituted for another, the demands for the two kinds of good will be interrelated by the fact that customers can trade off one good for the other if it becomes advantageous to do so . </P> <P> An increase in price (ceteris paribus) will result in an increase in demand for its substitute goods . If two goods have a high substitutability, the change in demand will be much greater . Thus, economists can predict that a spike in the cost of a particular brand of detergent is likely to result in a large change in demand for other brands, whereas a change in the price of pencils will have a much smaller effect on the demand for other stationery, such as pens on legal documents or pencils on most high - school maths homework . </P>

Which of the following is the best example of a pair of substitute goods