Final answer:
Glenda's elastic demand for cranberry juice is due to the a. availability of substitutes, as she easily switches to drinking grape juice when the price of cranberry juice increases.
Step-by-step explanation:
Glenda's switch from drinking cranberry juice to the less expensive grape juice when the price of cranberry juice increased is an example of the substitution effect. This effect occurs because consumers will opt for a substitute if the price of a good they usually buy goes up. In this scenario, the availability of a substitute, grape juice, is what allows Glenda to maintain her consumption habits without having to pay the higher price for cranberry juice.
Therefore, Glenda's elastic demand is due to availability of substitutes. This is because she has an alternative option that is comparable in satisfaction but more affordable. The existence of close substitutes like grape juice in relation to cranberry juice makes the demand for cranberry juice elastic, as consumers can easily switch between the two based on their prices.