根据短文内容，从方框中选择适当的句子填空，使短文意思完整、通顺。

     ____ Some people climbed the highest mountains, some traveled to unknown parts of the world, and some sailed small boats across the largest sea. Now some people begin to look for new excitement(刺激).

     ____ Bungee jumping only lasts a few minutes or even seconds. You jump from a high place, 200 meters above the ground, and there is a rubber band tied to your legs. When you jump down, the rubber band pulls you up. About 2,000,000 people around the world have tried bungee jumping.

     ____ Some scientists (科学家) say that it is because modem life has become safe and it is not interesting. ____ They had to go out and look for food, and life was like a fight but was interesting.

     ____ They live and work in safe places, buy food in shops, and there are doctors and hospitals to look after them if they become ill.

已知a∈R，试判断方程（a﹣1）x+y=3是什么方程．

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