One day in 1963, a dolphin named Elvar and

题目：

One day in 1963, a dolphin named Elvar and a famous astronomer, Carl Sagan, were playing a little game. The astronomer was visiting an institute which was looking into the way dolphins communicate with each other. He was standing at the edge of one of the tanks where several of these highly intelligent, friendly creatures were kept. Elvar had just swum up alongside him and had turned on his back. He wanted Sagan to scratch his stomach again, as the astronomer had done twice before. But this time Elvar was too deep in the water for Sagan to reach him Elvar looked up at Sagan, waiting. Then, after a minute or so, the dolphin leapt up through the water into the air and made a sound just like the word "More!"
The astonished astronomer went to the director of the institute and told him about the incident. "Oh, yes. That’s one of the words he knows," the director said, showing no surprise at all.
Dolphins have bigger brains in proportion to their body size than humans have, and it has been known for a long time that they can make a number of sounds. What is more, these sounds seem to have different functions, such as warning each other of danger. Sound travels much faster and much further in water than it does in air. That is why the parts of the brain that deal with sound are much better developed in dolphins than in humans. But can it be said that dolphins have a "language" in the real sense of the word Scientists don’t agree on this.
A language is not just a collection of sounds, or even words. A language has a structure, or what we call a grammar. The grammar of a language helps to give it meaning. For example, the two questions "Who loves Mary" and "Who does Mary love" mean different things. If you stop to think about it, you will see that this difference doesn’t come from the words in the question but from the difference in structure. That is why the question "Can dolphins speak" can’t be answered until we find out if dolphins not only make sounds but also arrange them in ways which affect their meaning.

The sounds we call words can be called a language only if ______.

A．each sound has a different meaning

B．each sound is different from the other

C．there is a system of writing

D．they have a structure or grammar