阅读下列短文，从每题所给的A、B、C、D四个选项中，选出最佳选项。
A
For the first time in the Pew Research Center's history, social media have replaced print newspapers as Americans' main news source, with 20 percent of adults surveyed reporting they rely on social media for the latest updates. In contrast, just 16 percent consider print newspapers as their most frequented medium.
These findings reflect the much－discussed decline of print media, as well as the rise of digital news sources. Still, Pew's researcher Elisa Shearer notes that television remains dominant (占优势的) across all mediums, with 49 percent of respondents—down 8 percent from 2016—reporting they learn the news mainly by watching local, cable and network news shows. News websites finish second overall, claiming the attention of 33 percent of Americans, an increase of 5 percent since 2016. Radio comes in third at 26 percent, followed by social media and, lastly, print newspapers.
Pew's director of journalism research, Amy Mitchell, writes that the new findings, which are based on a survey of 3，425 U．S. adults conducted between July 30 and August 12, 2018, point toward Americans' preference for watching rather than reading or listening to the news. Television remains the preferred viewing platform among watchers, but a growing minority (20 percent) say they prefer streaming news videos online.
Unsurprisingly, consumption habits varied dramatically according to age. About 80 percent of individuals aged 65 and older regard television as their most frequented news source, compared to just 16 percent of those aged 18 to 29 and 36 percent of those aged 30 to 49.
Younger Americans also tend to draw on a variety of platforms. Shearer writes that no more than half of those aged 18 to 29 or 30 to 49 find their news through a single medium, instead of opting for a mixture of news websites, social media, TV, radio and—in fewer numbers—print media.
21．What's the purpose of writing this text?
A．To explain the decline of print media.
B．To stress the influence of social media.
C．To show some of Pew Research Center's findings.
D．To reveal Americans' changes in consumption habits.
22．Why do Americans choose television as the main news source?
A．Because print newspapers are less preferred.
B．Because Americans prefer watching the news.
C．Because television is more convenient to watch.
D．Because America has a large ageing population.
23．How does the passage mainly develop?
A．By analyzing data. B．By providing examples.
C．By explaining causes and effects. D．By making classification.
