Passage Three Questions 41 to 45 are based on the following passage. The idea of using radio for broadcasting to mass audiences was formed in 1916 by an executive of the American Marconi Company, David Sarnff. His superiors were doubtful about his idea to "make radio a household utility (事业), (so that) by purchase of a‘radio music box’, the audience could enjoy lectures, music recitals (演奏会),etc. and his proposal was neglected. Four years later the American engineer .Frank Conrad, an employee at Westinghouse Electric Corporation, attracted considerable attention when a local newspaper reported on the growing audience listening on crystal radio sets to his evening and weekend broadcasts; a local music store had provided records to play on the Victrola, and Conrad and his family served as disc jockeys (唱片音乐节目广播员). Westinghouse vice president Harry Davis asked Conrad to build a more powerful transmitter in time to announce the outcome of the next U. S. presidential election. Conrad completed his assignment, and on November 2, 1920, station KDKA in Pittsburgh, Pennsylvania ,broadcast the announcement that Warren G. Harding had been elected president. About 1 000 people heard this first news broadcast. Radio communicated news much faster than did newspapers, and because crystal sets were easy to build and inexpensive, radio expanded rapidly in the following years .To stimulate the sale of radio sets, equipment manufacturers provided transmitting facilities. Singers, comedian, and entire orchestras volunteered their services for publicity. The ual financial basis of the new industry, however, was still unclear. One group in New York City tried to seek contributions from listeners; others urged that private foundations support radio .stations as a public service. In August 1922 the first commercial radio advertisement was broadcast on WEAF (now WNBC) in New York City. In 1926, when about 5 million homes had radios, the Radio Corporation of America (RCA), in cooperation with the American Telephone Telegraph Company, established the first commercial radio network. In the 1920s radio was established as a new mass medium and a practicable industry, and it became a national forum for news and popular culture. After 1920, radio expanded rapidly because ______.