Final answer:
An obvious substitute for ballpoint pens with blue ink includes ballpoint pens with black ink or pencils, making the demand for blue ink pens elastic due to easy substitution.
Step-by-step explanation:
An obvious substitute for ballpoint pens with blue ink would be ballpoint pens with black ink or any other writing instruments like pencils or gel pens. These substitutes contribute to the elastic demand for blue ink ballpoint pens, as consumers can easily switch to other colors or different types of pens or pencils without significantly affecting their ability to write. The existence of these substitutes means that if the price for blue ink pens were to rise, consumers could easily choose an alternative like a black ink pen without any real impact on their writing needs. This is why the demand for ballpoint pens with blue ink is considered elastic: because there are readily available and acceptable substitutes that do not dramatically alter the consumer’s experience or utility.