Why A?

@Marissa-Avnaim, The passage tells us that the cod estimate is calculated by averaging two estimates - sampling-based and commercial-tonnage. Over the past decade, the commercial tonnage estimate has been increasing markedly, by about the same amount as the sample-based estimate has been decreasing. Since one of these estimates has increased by about the same amount as the other one has decreased over the last 10 years, the net change is near zero, and the average of these two estimates must be about the same as ten years ago. Does this make sense? Let me know if you have any further questions.