Which of the following terms-of-trade concepts is calculated by dividing the change in a country's export price index by the change in its import price index between two points in time, multiplied by 100 to express the terms of trade in percentages?
【单选题】----I’m sure Kim will win the first prize in the final. -----I think so. He ______ for it for months. A .is preparing B. was preparing C. has been preparing D. had been preparing