简答题:根据下面资料，回答46-50题
A natural resource or resource is usually defined as anything obtained from the physical environment to meet human needs. Some resources are available for use directly from the environment. Examples include solar energy, fresh air, rainwater, fresh water in a river or stream, and naturally growing edible plants. Other resources, such as oil, iron, groundwater, fish, and game animals, are not directly available for our use. (46) Whether these and other materials in the environment are considered to be human resources depends on a combination of human ingenuity, economics, and cultural beliefs.
(47) Human ingenuity enables us to develop scientific and technological methods for finding, extracting , and processing many of the earth's natural substances and converting them to usable forms. Groundwater found deep below the earth's surface was not a resource until we developed the technology for drilling a well and installing pumps and other equipment to bring it to the surface. Fish and game animals are not a resource unless we have some way of catching and (in most cases) cooking them. Petroleum was a mysterious fluid until humans learned how to locate it, extract it, and refine it into gasoline, home heating oil, road tar and other products. (48) Cars, television sets, tractors,and other manufactured objects are available only because humans developed methods for converting an array of once useless raw materials from the earth's crust into useful forms.
Economics also determines whether something is classified as a resource or a potential resource. Some known deposits of oil, coal, copper, and other potentially useful materials are located so far beneath the earth's surface or in such low concentrations that they would cost more to find, extract, and process than they are worth. (49) In the future, however, their prices may rise due to their scarcity, or cheaper, more efficient mining and processing technologies may be developed, converting these potential resources to actual resources.
Cultural beliefs can also determine whether something is considered a resource. For example, protein-rich grasshoppers and other insects are considered food resources in some parts of Africa, but are viewed with disgust as sources of food in the United States and in most MDCs. In some cultures, religious beliefs prohibit the use of pork or other types of food resources. (50) The perceived or actual degree of risk involved in using a resource such as nuclear power can also play a role in whether or how widely it is used.
第（49）题答案为