中四econ demand and supply 一問

Competitive demand: the products are substitutes for each other. A rightward shift in the demand curve of one product will result in a leftward shift of the demand curve of the other product. This is most evident in cases where consumers switch to a cheaper commodity to maximise their satisfaction using their fixed disposable income e.g. chicken or duck. However, competitive supply means producers can produce either of two or more products, so if they produce more of one, they will produce less of another. Hence, their demands are inversely proportional to each other. For example, chicken and eggs. Therefore, I think 私人樓and居屋 are COMPETITIVE DEMAND as to certain extent, they are substitutes.