Answer:
Elastic demand
A heart valve
Explanation:
A good with many close substitutes will have a highly elastic demand. This is because an increase in the price of the good will causes the consumers to purchase one of its cheaper substitutes.
If both a diamond necklace and a heart valve for heart attack victims are priced the same, the price elasticity for the heart valve will be lower. This is because the diamond necklace is a luxury good but the heart valve is necessary for the survival of the victim.