题目原文：
Starting in the 1960s and continuing until the 1980s, sailors in Russian submarines patrolling the North Atlantic and Arctic Ocean would occasionally hear strange sounds. These underwater noises reminded the submarine crews of frog croaks, so they called the sounds “quackers” (from the Russian word for frog sounds). The sources of the sound seemed to be moving with great speed and agility; however, the submarines’ sonar (a method of detecting objects underwater) was unable to detect any solid moving objects in the area. There are several theories about what might have caused the odd sounds.
The first theory suggests that the strange noises were actually the calls of male and female orca whales during a courtship ritual. Orca whales are known to inhabit the areas where the submarines were picking up the bizarre noises. Orcas have been studied extensively, and the sounds they make when trying to attract a mate are similar to those that the submarines were detecting.
A second idea is that the sounds were caused by giant squid. Giant squid are giant marine invertebrates that live deep in the ocean and prey on large fish. They are difficult to detect by sonar because they have soft bodies with no skeleton. Not much is known about giant squid behavior, but their complex brains suggest they are intelligent animals. It is possible they have the ability to emit sound, and perhaps they approached the submarines out of curiosity.
A third theory suggests the Russian submarines were picking up stray sounds from some military technology, like another country’s submarines that were secretly patrolling the area. Perhaps the foreign submarines did not register on the sonar because they were using a kind of technology specifically designed to make them undetectable by sonar. The strange froglike sounds may have been emitted by the foreign submarines unintentionally.
题目音频：
学术写作：
教授：我们和其他国家的人的差异正在变大还是变小？
Andrew：变小了。
Claire：变大了。
Doctor Achebe:With so many opportunities to connect nowadays, some people believe that no matter which country we come from, we have more similarities than we have differences. However, others have observed that differences between themselves and people from other countries are growing. In your view, are differences between people from different countries increasing, or are they diminishing? Why?
Andrew: I think the differences among us are decreasing as our exposure to products from other countries grows. We can enjoy food from any country. We can watch movies made in faraway places. We can go to a store and buy authentic goods from around the world. These things become part of our normal life and make our differences seem smaller.
Claire: I understand what Andrew is saying, but for some people, being exposed to differences makes them retreat into their own corner even more. Look at social media. We are exposed to many different people and ideas, but a lot of people talk to only those they agree with and ignore the rest. This only makes the differences between people increase.
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