The correct answer is C. Large number of substitutes.
The price elasticity of demand is a measure of how responsive consumers are to changes in the price of a good. A good with a high price elasticity of demand means that consumers are very sensitive to changes in price, and will buy much less of the good if the price goes up. A good with a low price elasticity of demand means that consumers are not very sensitive to changes in price, and will not buy much less of the good even if the price goes up.
There are a number of factors that can affect the price elasticity of demand for a good. One of the most important factors is the number of substitutes available. If there are many substitutes available, consumers will be more likely to switch to another good if the price of one good goes up. This means that the demand for the good will be more elastic.
Another factor that can affect the price elasticity of demand is the necessity of the good. If a good is a necessity, consumers will be less likely to switch to another good if the price goes up. This means that the demand for the good will be less elastic.
The price elasticity of demand can also be affected by the time horizon. In the short run, consumers may not have time to adjust their consumption habits to changes in price. This means that the demand for a good may be less elastic in the short run than in the long run.
In conclusion, the factor which generally keeps the price-elasticity of demand for a good high is a large number of substitutes.