Detailed Instructions: In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.
See one example below:
Problem: Article: There are a lot of public libraries in Britain. These libraries have more than one hundred and fifteen million books, not including those libraries of public schools. Public libraries not only lend books, music records but also help children, patients in hospitals and others to get special play readings, film shows and so on. The first public libraries were founded in about the year 1850. Today's public libraries usually have a reading-room, a lending room and a reference room. The greatest and most famous library in Britain is that of the British Library in London. It was founded in 1973 and has over 10, 000, 000 books. It has nearly every magazine, newspaper and book published in Britain and other parts across the world.
Answer: 10,000,000 or less.
Solution: How many books are there in the British Library?
Explanation: In the Article, 10,000,000 is mentioned for the number of books available in the British Library, so this is a good question.

Problem: Article: Machu Picchu is considered to be a truly extraordinary archaeological discovery and is one of South America's largest tourist sites. As many as one thousand visitors visit the ancient Incan ruins daily. What draws vacationers and adventurers to this remote location high in the Andes Mountains of Peru ? A large part of the attraction is the mysteries that surround Machu Picchu. For example, Machu Picchu is considered "the lost city" of the Incas because it was never found by Spanish invaders and was known to only a few local peasants for almost four hundred years. The "city in the clouds", Machu Picchu's lasting beauty comes from the careful blending of its striking architecture with its mountaintop environment. Its greatest skill shows are in the stone structures, which equals that of the ancient Egyptians. The stone blocks were so appropriately cut that they fit together without the need for mud. The largest stones at Machu Picchu weigh as much as fifteen tons! It has been difficult to explain how the huge blocks were transported. The buildings, including many that have survived lots of earthquakes, contain stones which _ so perfectly that a knife cannot be forced between them. When compared to other ancient civilizations, it is difficult to understand how Machu Picchu could have been missed by the Spanish and lost from history for four hundred years. For many of the mysteries of Machu Picchu being unsolved, it is clear why Machu Picchu remains "the most famous ruins in all of South America".
Answer: Machu Picchu is the largest stone city of the world.
Solution:
Which of the followings is NOT a mystery of Machu Picchu?