B "As I stood in front of the grave (墓) of President Richard Nixon, I was thinking about the time 25 years ago when this president helped bring the United States and China closer together. Young people of our two countries should help this relationship grow." This remark was made by a Shanghai student when speaking to his fellow students at the Nixon Library in California, U.S.A.. He was one of 80 middle school students from China attending a month long "Youth Summit". The Summit was to mark the 25th anniversary (周年) of President Nixon’s journey to China, which was the turning point in China-U.S. relations. The Youth Summit was aimed at increasing understanding and friendship between young students of the two countries through visits and discussions. Seventy-five American students were selected to visit China. They also visited the Nixon Library on July 21 before leaving for Beijing the next day. The head of the Library said he was pleased to see the American and Chinese students talking and laughing together. One Chinese student said, "I didn’t find it particularly difficult to talk with Americans. We have our differences, but we have a lot in common. Dialogue is good for us." The words "Youth Summit" refer to ______.

A. visits to the Nixon Library
B. the Chinese students’ visit to the U.S.
C. a meeting discussing relations between China and the U.S.
D. activities to strengthen the ties between the Chinese and American students