During the 20th century things such as nuclear power, personal computers, airplanes, cars, rockets, submarines, antibiotics, tvs, ratios, etc. all came about. Clearly multiple means for transportation that were invented during that time are still very important in our daily lives these days. Not to mention the ways that the creation of antibiotics changed the world of medicine. Each of these things were huge advances that help us even now, and continue to evolve as time goes on.
Alrighty, I don't know what point C is but I do know what opportunity cost is and will attempt to give you some more insight into the question. Opportunity cost is a trade off. It's what you give up in order to obtain something else. For example, you would pay more in order to have a car with more safety features. A higher price for the consumers in exchange for more safety.
Answer:
The elastic clause allows Congress to make laws it needs to carry out its powers.