She is widely seen as proof that good looks can last for ever. But, at nearly 500 years of age, time is catching up with the Mona Lisa. The health of the famous picture, painted by Leonardo da Vinci in 1505, is getting worse by the year, according to the Louver Museum  where it is housed. "The thin, wooden panel on which the Mona Lisa is painted in oil has changed shape since experts checked it two years ago," the museum said. Visitors have noticed 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. Nearly 6 million people go to see the Mona Lisa every year, many attracted by the mystery of her smile. "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 details, but less suited to looking at shadows. Da Vinci painted the smile in shadows." However, the actual history of the Mona Lisa is just as mysterious as the smile. 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 Louver 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. During World War II, French hid the painting in small towns to keep it out of the hands of German forces. It may add to the difficulty in repairing the painting that  _  .
experts haven't noticed changes in its shape as visitors do it is likely to be stolen again when it's under repair it is uncertain which country, Italy of France, should take charge experts aren't sure about the materials and the chemical state of its oil paint
Question: She is widely seen as proof that good looks can last for ever. But, at nearly 500 years of age, time is catching up with the Mona Lisa. The health of the famous picture, painted by Leonardo da Vinci in 1505, is getting worse by the year, according to the Louver Museum  where it is housed. "The thin, wooden panel on which the Mona Lisa is painted in oil has changed shape since experts checked it two years ago," the museum said. Visitors have noticed 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. Nearly 6 million people go to see the Mona Lisa every year, many attracted by the mystery of her smile. "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 details, but less suited to looking at shadows. Da Vinci painted the smile in shadows." However, the actual history of the Mona Lisa is just as mysterious as the smile. 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 Louver 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. During World War II, French hid the painting in small towns to keep it out of the hands of German forces. It may add to the difficulty in repairing the painting that  _  .
A. experts haven't noticed changes in its shape as visitors do
B. it is likely to be stolen again when it's under repair
C. it is uncertain which country, Italy of France, should take charge
D. experts aren't sure about the materials and the chemical state of its oil paint
Answer: