Available choices: 1). Veins and arteries.. 2). lack of backbone. 3). bruised brain stem. 4). critical organs. 5). closed circulatory system. 6). incomplete circulatory system. 7). characteristics. 8). sensory neurons.

You are presented with the quiz "vertebrates have a complete digestive system and a what?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "chordates have a complete digestive system and a closed circulatory system" and "vertebrates are members of of a larger group, the chordates ". So, what's the best answer to the question?
Answer:
5).