题目内容：
阅读文章，回答题Reading can be a social activity.Think of the people who belong to book groups.They choose books to read and then meet to discuss them.Now, the website BookCrossing.com turns the page on the traditional idea of a book group.
Members go on the site and register the books they own and would like to share.BookCrossing provides an identification number to stick inside the book.Then the person leaves it in a public place, hoping that the book will have an adventure, traveling far and wide with each new reader who finds it.
Bruce Pederson, the managing director of BookCrossing, says, “The two things that change your life are the people you meet and books you read.BookCrossing combines both.”
Members leave books on park benches and buses, in train stations and coffee shops.Whoever finds their book will go to the site and record where they found it.
People who find a book can also leave a journal entry describing what they thought of it.E-mails are then sent to the BookCrossing to keep them updated about where their books have been found.Bruce peterson says the idea is for people not to be selfish by keeping a book to gather dust on a shelf at home.zxx.k
BookCrossing is part of a trend among people who want to get back to the “real” and not the virtual(虚拟).The site now has more than one million members in more than one hundred thirty-five countries.
Why does the author mention book groups in the first paragraph? A．To explain what they are.
B．To introduce BookCrossing.
C．To stress the importance of reading.
D．To encourage readers to share their ideas.
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