More and more people around the world are joining in dangerous sports. Some people climbed the highest mountains; some traveled into unknown parts of the world; some sailed small boats across the largest sea. Now some people begin to look for new excitement.

    Bungee jumping (蹦极) and motorcycle racing (摩托车赛) are quite dangerous sports. Bungee jumping only lasts for a few minutes or even seconds. You jump from a high place, about 200 meters above the ground, and there is a rubber band (橡胶带子) 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.

    Why do people join in these dangerous sports? Some scientists say that it is because modern life has become safe and it is not interesting. In the past, people lived in danger. They had to go out and look for food, and life was like a fight but was interesting.

    Many people think that there is little excitement in life. 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.

去年夏季，我国部分地区降雨量较大，对农业生产及自然环境造成很大影响。下列相关叙述中，正确的是（    ）

南方地区农作物一年两熟至三熟的主要原因是（    ）

设a，b，c是某三角形的三边长，证明a²（b+c﹣a）+b²（c+a﹣b）+c²（a+b﹣c）≤3abc．