四棱锥P﹣ABCD的底面ABCD为正方形，PA⊥底面ABCD，AB=2，若该四棱锥的所有顶点都在同一球面上，且该球的表面积为$\frac{81\mathrm{\pi }}{4}$，则该棱锥的高为（　　）

    When I was a teenager I volunteered to work at the water station at a 10,000m race. My job was to 1 water to the runners. I remember being so 2 to see all the different kinds of people who passed by and grabbed a cup of water. Some ran past, some walked past and a few wheeled past. I saw so many types of people doing it. I thought maybe I could do it too! So the next year I 3 up for the race.

    That first 10,000m race was quite an4 I jogged, I walked, I jogged and I walked.5 I didn't know if I could finish. Then came a defining (决定性作用的)6.

    At one point near the end, a 70-year-old man ran past me, very, very fast, and I felt7 because I was 15 years younger than he but I couldn't even keep up with him. I felt8for a second.

    But then I 9something. He was running his race and I was running mine. He had 10 abilities, experience, training and goals for himself. I had mine. Remember my 11was only to finish.

    After a minute, it 12 me that this was a lesson I could draw from. I learned something about myself in that moment. I turned my embarrassment into13.

    I 14 that I would not give up on running races. In fact, I would run even more races and I would learn how to train and prepare 15and one day I would be one of those 70-year-old persons who were still running. As I crossed the finish line, I was proud of my16.

    In life we all have those moments where we 17 ourselves to others. It's only 18. Don't allow those moments to 19 you. Turn them into motivation and let them inspire you. With the proper preparation and training, you can improve your result to 20anything you want in life.

    Having a teenager at home can be a real headache. Many teenagers smoke, drink and even fight. They often do something dangerous.

    Earlier studies have shown that teenagers are more likely (可能) to make irrational decisions than people from any other age group, including children and adults. Is it that teenagers are too young to tell right from wrong? Not really. So what's the reason?

    According to Laurence, a teacher from a US university, the reason is that teenagers care a lot about how their peers (同龄人) view them—that is “peer influence”.

    As children enter their teenage years, they spend more time with their friends and classmates, and also they care more about what they think of them. This makes teenagers make decisions without thinking about the costs.

    In a test, a group of teenagers were asked to play a video driving game. When they played with their friends watching around them, they took more chances and drove more carelessly because that would increase their possibility of winning. But when they played alone, they drove more safely.

    Why do peers have such a big influence on teens' behavior?

As Laurence sees it, a teenager's brain is like a car with a good accelerator (油门) but a weak brake (刹车). The “accelerator” is fully developed by teenage years while the “brake” is still not fully developed. When teenagers are watched by their peers, they usually push hard on the accelerator. With their weak brake, it is likely that they are going to end up in an accident.

    But the good news is that a violent (暴力的)teenager doesn't necessarily become a violent adult. About two-thirds to three-quarters of violent youth grow out of it. “They get more self-controlled.”