单选题 Cork is a light brown material harvested from the cork oak tree (橡树). Cork is lightweight, strong and resistant to water. It is best known for keeping liquids from spilling. For thousands of years, people have used pieces of cork as closures for bottles. Cork is made of small cells filled with air. This structure makes it elastic, so it can be pressed but later return to its full shape. It is said that no technology has been able to copy this unusual material exactly. The cork oak tree is native to the western Mediterranean coast of Europe. The largest cork oak forests in the world are located inPortugal. In fact,Portugalproduces more than half of the world's cork supply. Cork oak trees have to be at least twenty-five years old before they are ready for harvest. Harvests only happen once every nine years. Cork is gathered by skillfully cutting off the outer layer of the tree with special knives. The harvest weakens the tree temporarily, but it soon starts to grow a new layer of bark. Because cork oak trees are not killed during harvest, they can live for as long as two hundred years. Also, used cork products can be recycled and used again. This makes cork a valuable renewable resource小题:Where are the largest cork forests located in