When carbon dioxide enters the ocean, 30% of it is absorved by the water. Then, it reacts with seawater, forming carbonic acid. This process is called Ocean Acidification. However, shell-building animals needs free carbonate ions to form their shells. As the Ocean Adification is in process, those free ions will be used to form carbonic acid too, so they won't be available anymore. Therefore, there will be a decrease in the carbonate available for shell-bulding animals. So, the correct answer is B. decreases.