TOEFL Listening: TPO-TOEFL阅读TPO - B5MR1Z6HGA691TJ4N$

According to paragraph 6, which of the following is one of the reasons that tunas are in constant motion? A. They lack a swim bladder. B. They need to suck in more water than other fishes do. C. They have large muscles for breathing. D. They cannot open their mouths unless they are in motion.