A、Through a wide variety of high-frequency sounds. B、Through a wide variety of low-frequency sounds. C、Through their varied swim

The vast oceans of the world are dark, deep and mysterious places where eyesight counts for little as soon as you venture very far beneath the surface.
    For humans, who live in a world dominated by visual stimuli, to exist in such conditions would be impossible. But for whales and dolphins that live in the ocean or, in the case of a few species, muddy rivers, the darkness is unimportant. What is important to them is sound.
    Sound is an efficient way to transmit and sense information, especially as it travels five times faster through water than through air. If humans shout to someone, it is unlikely that they will be heard a kilometre away. But if a whale shouts in an ocean channel, another whale may hear it tens, if not hundreds, of kilometres away.
    Whales and dolphins use sound in two ways: for communication and for echolocation. Dolphins and toothed whales communicate through a wide variety of high-frequency sounds.
    But as well as using sounds to communicate, toothed whales and dolphins also rely on echolocation to learn about their immediate environment, including prey that might be waiting nearby. They produce intense short broad-band pulses of sound in the ultrasonic range. These clicks are brief, typically less than one millisecond long — but they are repeated many times each second.
Questions 16 to 18 are based on the passage you have just heard.
16. For whales and dolphins, which sense is most important?
17. How much faster does the sound travel in the water than in the air?
18. How do the dolphins and toothed whales communicate?