The reading passage presents some possible explanations for the strange sounds. But the lecturer argues that none of these explanations are reasonable. Details are shown below.
First, the reading passage contends that the strange noises were the calls of ocra whales during a courtship ritual. But the lecturer disagrees with this opinion. The whales are usually on the surface of the ocean, so the sailors in submarines could not hear them. What's more, the whale should be detected by the sonar. That is to say the explanation of whales is not convincing.
Second, the reading passage claims that the strange sounds might come from the giant squids. But the lecturer refutes this view, and states that it cannot explain why the giant squids generated sounds from 1960s to 1980s, but they did not generate sounds later. If the giant squids were the source of the strange sounds, they should continue to generate the sounds. So the explanation of giant squids is also unconvincing.
Third, the reading passage insists that the strange sounds might come from another country's secret submarines. However, the lecturer challenges this viewpoint by saying that a manmade machine could not move so fast and that the engine would generate other noise, which was not detected. In addition, the lecturer maintains that even today people cannot make such a fast and quiet machine. So the military technology cannot answer the question, either.