A. a kind of invertebrates B. a kind of marine mammalsC……

Every silver lining has its cloud. At the moment, the world’s oceans absorb a million tonnes of carbon dioxide an hour. Admittedly that is only a third of the rate at which humanity dumps the stuff into the atmosphere by burning fossil fuels, but it certainly helps to slow down global warming. However, what is a blessing for the atmosphere turns out to be a curse for the oceans. When carbon dioxide dissolves in water it forms carbonic acid. At the moment, seawater is naturally alkaline -but it is becoming less so all the time.
The biological significance of this acidification was a topic of debate among scientists. Many species of invertebrate have shells or skeletons made of calcium carbonate. It is these, fossilized, that form rocks such as chalk and limestone. And, as anyone who has studied chemistry at school knows, if you drop chalk into acid it fizzes away to nothing. Many marine biologists therefore worry that some species will soon be unable to make their protective homes. Many of the species most at risk are corals.
The end of the Permian period, 252m years ago, was marked by the biggest extinction of life known to have happened on Earth. At least part of the cause of this extinction seems to have been huge volcanic eruptions that poured carbon dioxide into the atmosphere. But some groups of animals became more extinct than others. Sponges, corals and brachiopods were particularly badly hit.
Rather than counting individual species of fossils, which vary over time, paleontologists who study extinction usually count entire groups of related species, called genera. More than 90% of Permian genera of sponges, corals and brachiopods vanished in the extinction. By contrast, only half of the genera of mollusks and arthropods disappeared.
This is because mollusks and arthropods are able to buffer the chemistry of the internal fluids from which they create their shells. This keeps the acidity of those fluids constant. Sponges, corals and brachiopods, however, cannot do this.
The situation at the moment is not as bad as it was at the end of the Permian. Nevertheless, calculations suggest that if today’s trends continue, the alkalinity of the ocean will have fallen by half a pH unit by 2100. That would make some places, such as the Southern Ocean, uninhabitable for corals. Since corals provide habitat and food sources for many other denizens of the deep, this could have a profound effect on the marine food web.
No corals, no sea urchins and no who-knows-what-else would be bad news indeed for the sea. Those who blithely factor oceanic uptake into the equations of what people can get away with when it comes to greenhouse-gas pollution should, perhaps, have second thoughts.
The word "brachiopod" in the third paragraph means______.

A. a kind of invertebrates
B. a kind of marine mammals
C. a kind of colonial plants
D. a kind of aquatic vertebrates