Why does the aquarium trade contribute to the introduction of nonnative species?
a. exotic species can escape or be released into local ecosystems.
b. aquarium owners do not always understand how ecosystems interact.
c. nonnative fish are shipped worldwide for aquariums.
d. all of the above

The correct option is D.

The aquarium trade refers to the international trade that involves live fishes and other water animals. These water animals are usually collected live from their native regions and transported to other parts of the world, where they are not known. In the process of transporting these water animals, they can escape and become part of the local ecosystem, thus, establishing themselves in the areas they are not meant for. Aquarium trade is one of the major means by which non native fish species are introduced to different part of the world either intentionally or accidentally.

the supermassive black hole is in the middle of the galaxy.

"a.) well–developed vascular system."