If marginal profit is zero, then average profit is at a maximum

Everything else remaining the same, in the foreign exchange market, which of the following increases the supply of U.S. dollars?

A) The U.S. interest rate rises. B) The U.S. interest rate differential increases. C) The expected future exchange rate rises. D) The European interest rate rises. E) The exchange rate falls.

In the above figure, when the quantity equals 400 pretzels

A) consumers are willing to pay $2 for the 400th pretzel. B) producers are willing to supply 400 pretzels for $3. C) producers are willing to supply 400 pretzels for $2. D) the marginal benefit is greater than the marginal cost.