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.

Let me give you an example: 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.
The answer to this example can be: How many books are there in the British Library?
Here is why: In the Article, 10,000,000 is mentioned for the number of books available in the British Library, so this is a good question.

OK. solve this:
Article: From furniture to jewelry, from the medical industry to civil engineering,three--dimensional printing can turn all sorts of ideas into objects. "I wanted to print large objects such as buildings,"said Behrokh Khoshnevis,professor of the University of Southern California.He planned to construct a house in 24 hours by applying 3D printing techniques,but he admitted there would be difficulties in meeting his goal. "For example,if you run out of ink while printing on paper,you will throw that paper away.But you cannot throw half completed buildings away if your material gets stuck,"he said.Although the printed house is not available to visit,there is a chance for people in Shanghai to see items printed in 3D. The Belgium-based company Materialize NV is currently holding a 3D printing exhibition in Shanghai that started in late 2012 and will continue until the middle of this year.The company is eager to show Asia what is possible with 3D printing. "All the pieces shown here are what cannot be made through traditional techniques. A good example of this customization is hearing aids More than l0 million people already use 3D printed hearing aids.all made according to the size of their ear."said Wim Michiels, executive vice-president at Materialise NV. "You can produce anything whenever and wherever you like.You don't need to set-up production lines for pieces that only need a limited amount of product.Setting up a production Line is a tot of work, takes a long time and is very expensive for a limited number of pieces" said Michiels. Kim Francois,managing director of Matcrialise China,said their European and American clients have at ready made the next step toward additive manufacturing with small-series production such as hearing aids.
Answer: Material.
Answer:
According to Khoshnevis,what is the main difficulty in printing buildings?