单选题 .  ①The disappearance of Steller's sea cow from the Bering and Copper islands by 1768 has long been blamed on intensive hunting. ②But its disappearance took only 28 years from the time Steller first described the species, a remarkably short time for hunting alone to depopulate the islands, especially given the large populations initially reported. ③However, by 1750, hunters had also targeted nearby sea otter populations. ④Fewer otters would have allowed sea urchin populations on which the otters preyed to expand and the urchins' grazing pressure on kelp forests to increase. ⑤Sea cows were totally dependent on kelp for food, and within a decade of the onset of otter hunting, Steller noted that the islands' sea cows appeared malnourished.43.  Which of the following can be inferred from the passage about kelp forests in the Bering and Copper islands between 1750 and 1768? ______