Đọc đoạn văn sau và chọn đáp án đúng cho mỗi câu hỏi : 

Species that belong to an area are said to be native species. Typically, they have been part of a given biological landscape for a long period, and they are well adapted to the local environment and to the presence of other native species in the same general habitat. Exotic species are interlopers, foreign elements introduced intentionally or accidentally into new settings through human activities. In one context an introduced species may cause no obvious problems and may, over time, be regarded as being just as "natural" as any native species in the same habitat. In another context, exotics may seriously disrupt delicate ecological balances and create a cascade of unintended consequences. The worst of these unintended consequences arise when introduced species put native species in destruction by preying on them, altering their habitats, or out-competing them in the struggle for food resources. Although biological introductions have affected environments the world over, the most destructive, effects have occurred on islands, where introduced insects, cats, pigs, rats, mongooses, and other nonnative species have caused the grave endangerment or outright extinction of literally hundreds of species during the past 500 years. One of other reason to cause species extinction is overexploitation. This word refers to the utilization of a species at a rate that is likely to cause its extreme endangerment or outright extinction. Among many examples of severe overexploitation, the case of the great whales stands out in special relief. By the middle of the 20th century, unrestricted whaling had brought many species of whales to incredibly low population sizes. In response to public pressure, in 1982 a number of nations, including the USA, agreed to an international moratorium on whaling. As a direct result, some whale species which are thought to have been on extinction's doorstep 25 years ago have made amazing comebacks, such as grey whales in the western Pacific.  Others remain at great risk. Many other species, however, continue to suffer high rates of exploitation because of the trade in animal parts. Currently, the demand for animal parts is centered in several parts of Asia where there is a strong market for traditional medicines made from items like tiger bone and rhino horn.

According to the second paragraph, by the middle of the 20th century ___.