Micro MC 問題

For B, the fact that people own significantly different quantities of the same good does not imply the need of exchange.
Imagine you have one can of drink, while your friend has ten. If one can is just enough for you to get enough satisfaction (utility) , then why should you exchange for more cans?
Exchange does not occur when you DO NOT NEED this good, while the difference in quantity cannot imply that you need this good.

For C, we value what we get more than what we give.
It follows immediately from the celebrated cost-benefit principle.
We value what we get <== Benefit
What we give <== Cost
By the cost-benefit principle, we will carry out an action if BENEFIT > COST
That's why exchange will occur, only when we know exchange can bring more satisfaction to us

For answer B
B is incorrect because wxchange can occur even if everyone possesses the same amount of the same good . Exchange occurs as long as their marginal use values on the good people possess are different .

For answer C
To buyers , the amount of money they give uo is less than the marginal use value of the good they buy . To seller , the amount of money they receive is more than the marginal use value of the good they sell