The article is mostly about __A．how sound is produ

单项选择题

Questions 41 to 44 are based on the following passage: By direct observations and many experiments, biologists have discovered that practically all animals have some sense of hearing or vibration. Earthworms feel vibrations in the soil, fish can be trained to respond to certain tones, male mosquitoes are attracted by the sound of the female, and frogs will respond to a tape recording of their own voices. The inner ear is composed of delicate membranes which bear dense patches of specialized cells called maculae. Each of these collections of cells can carry a message to the brain. What message is carried by a macula depends upon how it is affected. The message which is carried is not, however, always connected with the hearing sense. For instance, a certain kind of tadpole can tell the depth of the water it is swimming in by the pitch of a tone which is produced by its own lungs. In the human and all other mammals, the macula has developed into an organ which can easily be seen. This organ is called the cochlea. This spiral shaped organ contains the macula itself and it is called "organ of Corti" after its discoverer. If you have ever seen a snail shell, you know how a cochlea looks. When sound waves enter the cochlea, which is really a tube coiled around, they set a membrane into a back and forth motion and cause a new wave. This is something like the way in which high and low sounds are produced by a flute or whistle. The high sounds are produced when the air is prevented by the holes from going through, while the low sounds are produced by allowing more of the air to pass. All this is what produces the differences between high and low sounds. The loudness of a sound is evidently produced by how much the membrane is cause to move. Whether or not hearing is really produced in all animals by the effect of pressure is not definitely known by scientists as yet. We do know, however, that nature has set up some very delicate hearing mechanisms for its creatures. Scientists must explore much further for more knowledge about how animals use their ears.

The article is mostly about __

A．how sound is produced
B．learning by observing
C．the workings of the inner ear
D． outer ear formation in various animals