Answer:
C. the euro has gained strength against the dollar.
Explanation:
If Blake gives up more U.S. dollars in exchange for less euros every time, this means that the exchange rate is increasing. The exchange rate is the price of a currency in exchange for another. So, if the price is increasing that means that euro is getting stronger than dollars, which is contrary to affirmation A and B, but is affirmation C. This could be because the demand for euro is stronger than the demand for dollars, which is contrary to affirmation D.