性染色体存在于（　　）

阅读下面短文, 掌握其大意, 然后从各题所给的A、B、C、D四个选项中选出最佳选项。

    A boy thought that he was the most unhappy person in the world. In his opinion, he was unlucky because almost everything 1always happened to him. He wanted to find the 2 to happiness. He walked a long way and finally came to a beautiful palace. There, he met the king.

    After learning 3 the boy came, the king suggested that he should look around the palace and come back in two hours. “At the same time, I want you to hold this spoon 4 milk,” said the king. “As you walk around my palace, carry this spoon 5 without spilling the milk.”

    The boy kept his eyes fixed on the spoon and began to walk around the golden palace. After two hours, he came back to the king. “Well,” asked the king. “Did you 6 the wonderful paintings in my dining hall? Did you enjoy the colorful flowers in the garden?” The boy felt sorry because he saw 7.  He only paid attention to the milk.

    "Then go back and you can see the beauty of my world.” said the king. This time the boy saw all of the works of art on the walls. He saw the great gardens and hills all around. He had a great time 8 he was walking around the palace. When he 9, he described everything he saw.

     "But where is the milk?” asked the king. Looking down at the spoon he held, the boy saw the milk was gone. “Well, there is only one piece of 10 I can give you,” said the king. “The secret to happiness is to see all the beauty of the world and also keep an eye on the milk on the spoon.”

根据短文理解，选择正确答案。

    Grammar is the system of a language. People sometimes describe grammar as the “rules” of a language; but in fact no language has rules. If we use the word “rules”, we suggest that somebody created the rules first and then spoke the language, like a new game. But languages did not start like that. Languages started by people making sounds which evolved (逐渐发展成) into words, phrases and sentences. No commonly-spoken language is fixed. All languages change over time. What we call “grammar” is simply a reflection (反映) of a language at a particular time.

    Do we need to study grammar to learn a language? The short answer is “no”. Very many people in the world speak their own, native language without having studied its grammar. Children start to speak before they even know the word“grammar”. But if you are serious about learning a foreign language, the long answer is “yes, grammar can help you to learn a language more quickly and more efficiently.” It's important to think of grammar as something that can help you, like a friend. When you understand the grammar (or system) of a language, you can understand many things yourself, without having to ask a teacher or look in a book.

    So think of grammar as something good, something positive, something that you can use to find your way---like a signpost(路标) or a map.

    Except invented languages like Esperanto（世界语）. And if Esperanto were widely spoken, its rules would soon be very different.