A
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，travelling far and wide with each new reader who finds it.
★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.
B [作者在第一段中提出读书也是一种社交活动，那些参加读书小组的人经常在一起阅读讨论所读内容，增强相互之间的理解。接着在最后一句提出BookCrossing.com，说明作者提到读书小组的目的正是为了介绍BookCrossing.com这个网站。故B项正确。]
