Everyone knows that if he shouts near a wall or a mountainside, an echo will come back. The further off this solid obstacle, the longer time it will take for the return of the echo. A sound made by tapping on the main body of a ship will be reflected from the sea bottom, and by measuring the time interval between the taps and the receipt of the echoes the depth of the sea at that point can be calculated. So was born the echo-sounding equipment, now in general use in ships. Every solid object will reflect a soundThe main purpose of this passage is to____!

Not all sounds made by animals serve as language, and we have only to turn to that extraordinary discovery of echolocation (回声定位) in bats to see a case in which the voice plays a strictly practical role. To get a full appreciation of what this means we must turn first to some recent human inventions. Everyone knows that if he shouts near a wall or a mountainside, an echo will come back. The further off this solid obstacle, the longer time it will take for the return of the echo. A sound made by tapping on the main body of a ship will be reflected from the sea bottom, and by measuring the time interval between the taps and the receipt of the echoes the depth of the sea at that point can be calculated. So was born the echo-sounding equipment, now in general use in ships. Every solid object will reflect a sound The main purpose of this passage is to____! A. describe that animals can make different sounds B. prove that animals voices can play practical roles C. inspire the readers to make more inventions D. startle the readers with some shocking facts