Which of the following best defines "medium of exchange"?

A. something with a fixed value that we use for exchange

B. a way to determine what is the best deal

C. the average value exchange of a single good or service

D. a monetary rate that stays constant

I find the first option best.

B. even if something is a best deal - we might not be able to get it, so exchange might not even take place.

C. this would not be a "medium"

D. - well, there isn't such a rate, is there?

So, answer A is really the best - and I think it's also a good definiton