Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions.
Together, the thousands of scientists and conservation organizations that contribute to the International Union for Conservation of Nature (IUCN) Red List and other systems of assessment provide the world's largest knowledge base on the global status of species. The aim of these systems is to provide the general public, conservationists, nongovernmental organizations, the media, decision makers, and policy makers with comprehensive and scientifically rigorous information on the conservation status of the world's species and the threats that drive the observed patterns of population decline. Scientists in conservation and protected area management agencies use data on species status in the development of conservation planning and prioritization, the identification of important sites and species for dedicated conservation action and recovery planning, and educational programs. Although the IUCN Red List and other similar species-assessment tools do not prescribe the action to be taken, the data within the list are often used to inform legislation and policy and to determine conservation priorities at regional, national, and international levels. In contrast, the listing criteria of other categorization systems (such as the United States Endangered Species Act, the CITES) are prescriptive; they often require that landowners and various governmental agencies take specific mandatory steps to protect species falling within particular categories of threat It is likely that many undescribed or unassessed species of plants, animals, and other organisms have become or are in the process of becoming extinct. To maintain healthy populations of both known and unknown species, assessments and reassessments are valuable tools. Such monitoring work must continue so that the most current knowledge can be applied to effective environmental monitoring and management efforts. For many threatened species, large well-protected conservation areas (biological reserves) often play major roles in curbing population declines. Such reserves are often cited by conservation biologists and other authorities as the best way to protect individual species as well as the ecosystems they inhabit. In addition, large biological reserves may harbor several undescribed and unassessed species. Despite the creation of several large reserves around the world, poaching and illegal trafficking plague many areas. Consequently, even species in those areas require continued monitored and periodic assessment.
4. The word 'they' in paragraph 1 refers to ____.
