阅读下面短文，掌握其大意，然后各题所给的A、B、C和D项中，选出最佳选项。

　　Electronic books have changed the way many people read for pleasure．Now online textbooks are changing the way some students 　　1　　 and some teachers teach．

　　More than 175, 000 students 　　2　　 the public schools in Fairfax County, Virginia, outside Washington．Last year, the school system used digital 　　3　　 in fifteen schools．This school year, middle schools and high schools changed from 　　4　　 to electronic textbooks in their social studies classes．

　　Luke Rosa is a history teacher at Falls Church High School．His 　　5　　 work on school laptop computers．He explains the idea to them this way："I mean, it's just like a 　　6　　 textbook, except it's got it all online．"

　　Peter Noonan, a leader of schools, says with electronic textbooks, publishers can quickly 　　7　　 the content with the latest information．He says："The world's changing 　　8　　．And the online textbooks can change right along with the events that are happening．" He says digital books also cost 　　9　　 than printed textbooks："Usually it's in the neighborhood of between fifty and seventy dollars to 　　10　　 a textbook for each student, which adds up to 　　11　　 eight million dollars for all of our students in Fairfax County．We 　　12　　 have purchased all of the online textbooks for our students for just under six million dollars．"

　　But the students also need 　　13　　 to the Internet when they are not at school．About ten percent of students in Fairfax County do not have a computer or online access at 　　14　　．Stephen Castillo is one of them．He has to go the public library, which has free 　　15　　．

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