Each year, an official estimate of the stock of cod in the Grand Banks is announced. This estimate is obtained by averaging two separate estimates of how many cod are available, one based on the number of cod caught by research vessels during a once-yearly sampling of the area and the other on the average number of tons of cod caught by various commercial vessels per unit of fishing effort expended there in the past year -- a unit of fishing effort being one kilometer of net set out in the water for one hour. In previous decades, the two estimates usually agreed closely. However, for the last decade the estimate based on commercial tonnage has been increasing markedly, by about the same amount as the samplingbased estimate has been decreasing. If the statements in the passage are true, which one of the following is most strongly supported by them?
A: The once-yearly sampling by research vessels should be used as the sole basis for arriving at the official estimate of the stock of cod.
B: The number of commercial vessels fishing for cod in the Grand Banks has increased substantially over the past decade.
C: Last year's official estimate was probably not much different from the official estimate ten years ago.
D: Twenty years ago, the overall stock of cod in the Grand Banks was officially estimated to be much larger than it is estimated to be today.
Choose A, B, C or D as your solution.
C