根据短文理解，选择正确答案。
A year ago, I paid no attention to English idioms( 习语 ), though my teacher emphasized(强调) the importance again and again. But soon, the importance of English idioms was shown in an amusing experience.
One day, I happened to meet an Englishman on the road, and soon we began to talk. As I was talking about how I was studying English, the foreigner seemed to be astonished. Gently shaking his head, shrugging his shoulders, he said, “You don't say!” “You don't say!” I was puzzled. I thought, perhaps this is not an appropriate(合适的) topic. “Well, I'd better change the topic.” So I said to him. “Well, shall we talk about the Great Wall? By the way, have you ever been there?” “Certainly, everyone back home will laugh at me if I leave China without seeing it. It was magnificent(宏伟的).” He was deep in thought when I began to talk like a tourist guide. “The Great Wall is one of the wonders in the world. We are very proud of it.” Soon I was interrupted again by his words “You don't say!” I couldn't help asking, “Why do you ask me not to talk about it?” “Well, I didn't request you to do so,” he answered, greatly surprised. I said, “Didn't you say ‘you don't say?'” Hearing this, the Englishman laughed to tears. He began to explain, “‘You don't say' actually means ‘really!'. It is an expression of surprise. Perhaps you don't pay attention to English idioms.” Only then did I know I had made a fool of myself. Since then I have been more careful with idiomatic expressions(习语表达). Remember -- what the English teachers said is always right to us students.
，函数
在
单调递减，则
（ ）