Introduction:
Price elasticity of demand measures the responsiveness of the quantity demanded of a good to a change in its price. It helps in understanding how sensitive consumers are to changes in price. A high price elasticity of demand indicates that consumers are highly responsive to price changes, while a low price elasticity of demand suggests that consumers are less responsive to price changes.

Factors affecting price elasticity of demand:

1. Availability of substitutes:
The availability of substitutes is a key factor that influences the price elasticity of demand. If there are many substitutes available for a particular good, consumers have more options to choose from. In such cases, if the price of the good increases, consumers can easily switch to the substitutes, reducing their demand for the original good. This makes the demand for the good more elastic. Conversely, if there are few or no substitutes available, consumers have limited options and are less likely to switch to other products, resulting in a less elastic demand.

2. Degree of necessity:
The degree of necessity of a good also affects its price elasticity of demand. Goods that are considered necessities, such as food, healthcare, and basic utilities, tend to have a relatively inelastic demand. Consumers are less likely to reduce their consumption of these goods even if their prices increase. On the other hand, goods that are considered luxuries or non-essential items have a more elastic demand. Consumers can easily reduce their consumption of these goods if their prices rise.

3. Proportion of income:
The proportion of income spent on a good also influences its price elasticity of demand. If a good represents a significant portion of consumers' income, they are more likely to be price-sensitive and demand for the good will be more elastic. On the other hand, if a good represents a small portion of consumers' income, they are less likely to be price-sensitive and demand for the good will be less elastic.

4. Time:
The time period considered also affects the price elasticity of demand. In the short run, consumers may have limited options to adjust their consumption patterns in response to price changes. Therefore, the demand for a good may be relatively inelastic in the short run. However, in the long run, consumers have more flexibility to make adjustments and find substitutes. This makes the demand for the good more elastic in the long run.

Conclusion:
Among the factors mentioned above, the availability of substitutes is the most significant factor that generally keeps the price elasticity of demand for a good high. When consumers have multiple substitutes to choose from, they can easily switch to alternatives if the price of the original good increases. This makes the demand for the good more elastic. Conversely, if there are limited substitutes available, consumers are less likely to switch to other products, resulting in a less elastic demand.