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Christian Eijkman, a Dutch doctor, left the Netherlands for the island of Java. Many people on the island had a disease called beri-beri. He was going there to try and find a cure.
At first, Eijkman thought some kind of germ (细菌) caused beri-beri. He raised some chickens. He didn't eat them, but made experiments on them. The local people were quite surprised at that. One day he noticed that his chickens became sick when they were fed the food most Javanese ate — refined white rice (精炼米). When he fed them with unrefined rice, also known as brown rice, they recovered. Eijkman realized that he had made an important discovery — that some things in food could prevent disease. These things were named vitamins. The Javanese were not getting enough vitamins because they had actually removed the part that contains vitamins. Later, other diseases were also found to be caused by the lack of vitamins in a person's food.
Today many people know the importance of vitamins and they make sure they have enough vitamins from the food they eat. If they don't, they can also take vitamin pills.
上任意一点，过C作⊙O的切线分别交PA，PB于D，E．若△PDE的周长为12，则PA的长为（ ）
