Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 24 to 33.Since the world became industrialized, the number of animal species that have either become extinct or have neared extinction has increased. Bengal tigers, for instance, which once roamed the jungles in vast numbers, now number only about 2,300. By the year 2025, it is estimated that they will become extinct. What is alarming about the case of ...

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 24 to 33.

Since the world became industrialized, the number of animal species that have either become extinct or have neared extinction has increased. Bengal tigers, for instance, which once roamed the jungles in vast numbers, now number only about 2,300. By the year 2025, it is estimated that they will become extinct. What is alarming about the case of the Bengal tiger is that this extinction will have been caused almost entirely by poachers who, according to some sources, are not always interested in material gain but in personal gratification. This is an example of the callousness that is contributing to the problem of extinction. Animals such as the Bengal tiger, as well as other endangered species, are valuable parts of the world's ecosystem. International laws protecting these animals must be enacted to ensure their survival - and the survival of our planet.

Countries around the world have begun to deal with the problem in various ways. Some countries, in an effort to circumvent the problem, have allocated large amounts of land to animal reserves. They then charge admission prices to help defray the costs of maintaining the parks, and they often must also depend on world organizations for support. This money enables them to invest in equipment and patrols to protect the animals. Another response to the increase in animal extinction is an international boycott of products made from endangered species. This has had some effect, but by itself it will not prevent animals from being hunted and killed.

What does the word 'this' in line 6 refer to in the passage?