Having no language infants cannot be told what they need to learn. Yet by the age of three they will have mastered the basic structure of their native language and will be well on their way to communicative competence. Acquiring their language as a most impressive intellectual feat is the rapid acquisition of grammar. Nevertheless, the ability of children to conform to grammatical rules is only slightly more wonderful than their ability to learn words. It has been reckoned that the average high school graduate in the United States has a reading vocabulary of 80,000 words, which includes idiomatic expressions and proper names of people and places. This vocabulary must have been learned over a period of 16 years. From the figures, it can be calculated that the average child learns at a rate of about 13 new words per day. Clearly a learning process of great complexity goes on a rapid rate in children. What is the main subject of the passage

单项选择题The American economic system is organized around a basically private enterprise, market-oriented economy in which consumers largely determine what shall be produced by spending their money in the market-place for those goods and services in competition with other businessmen, and the profit motive operating under competitive pressures, largely determines how these goods and services are produced. Thus, in the American economic system it is the demand of individual consumers, coupled with the desire of businessmen to maximize profits and the desire of individuals to maximize their incomes, which together determine what shall be produced and how resources are used to produce it. An important factor in a market-oriented economy is the mechanism by which consumer demands can be expressed and responded to by producers. In the American economy, this mechanism is provided by a price system, a process in which prices rise and fall in response to relative demands of consumers and supplies offered by seller-producers. The important factor in private-enterprise economy is that individuals are allowed to own productive resources (private property) , and they are permitted to hire labor, gain control over natural resources, and produce goods and services for sale at profit. In the American economy, the concept of private property embraces not only the ownership of productive resources but also certain rights, including the right to determine the price of product or to make a free contract with another private individual. The passage is mainly about______.

单项选择题Central Park, emerging from a period of abuse and neglect, remains one of the most popular attractions in New York City, with half a million out-of-towners among the more than 3 million people who visit the park yearly. About 15 million individual visits are made each year. Summer is the season for softball, concerts, and Shakespeare; fall is stunning; winter is wonderful for sledding, skating, and skiing; and spring-time is the loveliest of all. It was all planned that way. About 130 years ago Frederic Law Olmsted and his collaborator Calvert Vaux submitted their landscaping plan for rectangular parcel two miles north of the town’s center. The barren swampy tract, home for squatters and a bone-boiling works that made glue, was reported as a pestilential spot where miasmic odors taint every breath of air . It took 16 years for workers with pickaxes and shovels to move 5 million cubic feet of earth and rock, and to plant half a million trees and shrubs, making a tribute to nature—a romantic nineteenth-century perception of nature. What exists today is essentially Olmsted and Vaux’s plan, with more trees, buildings, and asphalt. Landscape architects still speak reverently of Olmsted’s genius and foresight, and the sensitive visitor can see the effects he sought. With what subject is the passage mainly concerned