Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
Example: In 1582 the united provinces invited francis, duke of anjou to lead them; but after a failed attempt to take antwerp in 1583, the duke left the netherlands again. after the assassination of william of orange (10 july 1584), both henry iii of france and elizabeth i of england declined the offer of sovereignty. however, the latter agreed to turn the united provinces into a protectorate of england (treaty of nonsuch, 1585), and sent the earl of leicester as governor-general. this was unsuccessful and in 1588 the provinces became a confederacy. the union of utrecht is regarded as the foundation of the republic of the seven united provinces, which was not recognized by the spanish empire until the peace of westphalia in 1648.
Example solution: In what year did francis, duke of anjou leave the netherlands?
Example explanation: The generated question is perfect. Because the question is based on second sentence of the input passage and its answer is 1583.

Problem: Most musicians agree that the best violins were made in Cremona, Italy, about 200 years ago. They even sound better than violins made today. Violin makers and scientists try to make instruments like the old Italian violins. But they aren't the same. Why are these old Italian violins so special? Many people think they have an answer. Some people think it is the age of the violins. But there is a problem here. Not all old violins sound wonderful. Only those from Cremona are special. So age cannot be the answer. Other people think the secret to those violins is the wood. The wood of the violin is very important. It must be from certain kinds of trees. It must not be too young or too old. Perhaps the violin makers of Cremona knew something special about wood for violins. But the kind of wood may not be so important. It may be more important to cut the wood in a special way. Wood for a violin must be cut very carefully. It has to be the right size and shape. The smallest difference will change the sound of the violin. Musicians sometimes think that this is the secret of the Italians. Size and shape may not be the answer either. Scientists make new violins that are exactly the same size and shape. But the new violins still do not sound as good as the old one. Some scientists think the secret may be the varnish , which covers the wood of the violin and makes it look shiny. It also helps the sound of the instrument. Since no one knows what the Italian violin makers used in their varnish, no one can make the same varnish today. There may never be other violins like the violins of Cremona. And there are not many of the old violins left. So these old violins are becoming more and more precious.

Solution: What is still unclear about Cremona violins according to the writer?