Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.

Q: During active transport, molecules move from an area of low concentration to an area of high concentration. This is the opposite of diffusion, and these molecules are said to flow against their concentration gradient. Active transport is called "active" because this type of transport requires energy to move molecules. ATP is the most common source of energy for active transport. As molecules are moving against their concentration gradients, active transport cannot occur without assistance. A carrier protein is always required in this process. Like facilitated diffusion, a protein in the membrane carries the molecules across the membrane, except this protein moves the molecules from a low concentration to a high concentration. These proteins are often called "pumps" because they use energy to pump the molecules across the membrane. There are many cells in your body that use pumps to move molecules. For example, your nerve cells (neurons) would not send messages to your brain unless you had protein pumps moving molecules by active transport. The sodium-potassium pump ( Figure 1.1) is an example of an active transport pump. The sodium-potassium pump uses ATP to move three sodium (Na+ ) ions and two potassium (K+ ) ions to where they are already highly concentrated. Sodium ions move out of the cell, and potassium ions move into the cell. How do these ions then return to their original positions? As the ions now can flow down their concentration gradients, facilitated diffusion returns the ions to their original positions either inside or outside the cell.

A: complete the following sentence. as molecules are moving against their concentration gradients, active transport cannot occur without __________.
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Q: If you want to become a fluent English speaker you should take some advice: There are four skills in learning English. They are reading, listening, speaking, and writing. The most important thing you must remember is that if you want to improve your speaking and writing skills you should first master the skills of reading and listening. Read as much as you can. But your reading must be active. It means that you must think about the meaning of the sentence, the meanings of the unfamiliar words, etc. There is no need for you to pay much attention to grammars or try to understand all the unfamiliar words you _ , but the fact that you see them for the first time and recognize them whenever you see them, for example in other passages or books, is enough. It would be better to prepare yourself a notebook so you can write down the important words or sentences in it. As for listening, there are two choices: besides reading, you can listen every day for about 30 minutes. You can only pay attention to your reading and become skillful at your reading, then you can catch up on your listening. Since you have lots of inputs in your mind, you can easily guess what the speaker is going to say. This never means that you should not practice listening. For listening you can listen to cartoons or some movies that are specially made for children. Their languages are easy. Or if you are good at listening you can listen to VOA or BBC programs every day. Again the thing to remember is being active in listening and preferably taking some notes. If you follow these pieces of advice, your speaking and writing will improve automatically, and you can be sure that with a little effort they will become perfect.

A: According to the author, which should you improve first among the four skills?
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Q: Terra-cotta Warriors As the greatest archaeological findings of the 20th century, Terra-cotta Warriors has a really long history.In 246 BC, Qin Shi Huang, later the first Emperor of all China, had begun to prepare for his mausoleum which took 11 years to finish to defend him in the afterlife.There are over 7,000 pottery soldiers, horses, bronze chariots, and even weapons found in it now.Terra-cotta Warriors was listed by UNESCO in 1987 as one of the world cultural heritages and it has made Xi'an a famous city for tourists. Big Wild Goose Pagoda The Big Wild Goose Pagoda has a history of over 1,300 years and is a typical ancient Chinese building.It is located in the south of Xi'an City.Because Master Xuanzang stored his classics brought from India in the pagoda, it is also a holy place for Buddhists.As a National Key Cultural Relic Preserve, it won the title of an AAAA Tourist Attraction as well. Xi'an Ancient City Wall As one of the landmarks of Xi'an, Ancient City Wall still stretches round the old city today, dividing the city into the inner part and the outer part.Ancient City Wall was originally built during the old Tang Dynasty (618 -- 907) and then enlarged by Zhu Yuanzhang, Emperor of the Ming Dynasty, thus forming the modern Xi'an City Wall.After the extension, the wall now stands 12 meters tall, 12-14 meters wide at the top and 15-18 meters thick at the bottom.

A:
Who are the text written for?
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