随着升学考的来临，我们的初中生活也即将结束。三年的初中生涯，总有一些难以忘怀的时刻。仔细阅读以下四段材料，从A、B、C、D中选择一个合适的标题。并完成第（2）小题。
A. An unforgettable teacher B. An unforgettable competition C. An unforgettable trip D. An unforgettable running exercise |
①
Hello, I'm Tom. I remember at one time, when we were running on the playground, I was too tired to go on. As I wanted to give up, some of my classmates cheered me up. My classmates' encouragements helped me finish the run. I'll never forget it.
②
Hello, I'm Mary. When I was in Grade Two, I didn't prepare well for an English contest. I said sorry to my teacher. He said: “Any opportunity is important. You should never give it up easily.” Later on, I made good use of every opportunity. I'll never forget him.
③
Hi, I'm Lucy. The time I traveled to Qingdao with my classmates was the most unforgettable experience. We swam in the sea and caught crabs (螃蟹) on the beach. We played games and enjoyed the beautiful scenery. I will never forget that trip.
④
Hi, I'm Jim. I took part in the red song competition with my classmates last year. Every day, we rehearsed(排练) very hard in order to get a good result. Because of our hard work, we achieved a good result in the competition.
素，可形成链式或环式结构，在一个二十三肽的化合物中最多含有肽键23个。 