68. The purpose of the two experiments is to .

　　A. explain when people can have a sixth sense

　　B. show how people act while being watched in the lab

　　C. study whether humans can sense when they are stared at

　　D. prove why humans have a sixth sense

　　【实战演练练习十五】

　　阅读下面短文，并将文后标有A-F 的句子（或段落）插入文章中标号为71-75的合适位置，使短文结构完整。其中有一个句子（或段落）是多余的。

　　She is widely seen as proof that good looks can last forever. But, at nearly 500 years of age, time is catching up with the Mona Lisa.

　　71

　　“The thin, wooden panel on which the Mona Lisa is painted in oil has changed shape since experts checked it two years age,” the museum said. 72 .

　　73 “It is very interesting that when you’re not looking at her, she seems to be smiling, and then you look at her and she stops,” said Professor Margaret Livingstone of Harvard University, “It’s because direct vision is excellent at picking up detail, but less suited to looking at shadows. Da Vinci painted the smile in shadows.”

　　74 Da Vinci himself loved it so much that he always carried it with him, until it was eventually sold to France’s King Francis I in 1519.

　　In 1911, the painting was stolen from the Louvre by a former employee, who took it out of the museum hidden under his coat. He said he planned to return it to Italy. The painting was sent back to France two years later.

　　75

　　A. Nearly 6 million people go to see the Mona Lisa every year, many attracted by the mystery of her smile.

　　B. However, the actual history of the Mona Lisa is just as mysterious as the smile.

　　C. Visitors have notice the changes but repairing the world’s most famous painting is not easy. Experts are not sure about the materials the Italian artist used and their current chemical state.