阅读理解
In Los Angeles, a waiter served a group of teens. The waiter remembered his experience as a young high school prudent. So he made sure the teens had a pleasant experience at the restaurant. The waiter made sure of it.
After serving all the teens doing a great job, he was left a tip（小费）,only $3.28. That was hardly enough to buy a drink at Roebucks.
The waiter went home. And he felt hatred for the students who did not know the value of hard work. They looked down on people like him. Or so he thought.
After several days, the waiter forgot about it. His life went on as usual. But ten days later, he received a letter from the teens.
“About a week and a half ago, on October 7, my three friends and I came to eat at this restaurant as our own homecoming celebration. It was an exciting experience for us to be here alone, and it was all new to us. And you were the best waiter we could ask for. You were kind, helpful, and didn't treat us like babies! So I want to say thank you for making our 'groan-up' experience so amazing and fun.
“I would also like to say sorry on behalf of(代表) my group. Since we were new to all of this, reasonable(合乎情理)，but we had completely forgotten(and didn't know, to be honest) what a tip even was, let alone how much to give. So we emptied our pockets, and all our money added up to $3.28.
“Having no idea of how small this really was, we left. Later, we realized our mistake and felt terrible. We knew we had to make it right.
“So together with this letter, you will find the correct 18% tip + extra for simply being amazing. Thank you for your help and patience and also thank you for making our night fun. Thank you!”
的小滑块静止于P点．已知滑块和半球形容器间的动摩擦因数为μ，滑块所受支持力大小为FN ，摩擦力大小为Ff ，弹力OP与水平方向的夹角为θ．下列关系正确的是（ ）
