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（2）小题

    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.

和田玉分布于昆仑山，是由大理岩与岩浆接触变质形成的玉石，其中产于山上原生矿处的称为山料，产于河床中磨圆程度好的称为籽料。和田玉属于（ ）

盛有NO 和  NO₂的混合气体的量筒倒立在盛有水的水槽中，过一段时间后，量筒内的气体体积减小为原来的一半 。则原混合气体中NO 和  NO₂的体积比是  （  ）

在等腰梯形$ABCD$中，已知$AB\parallel DC$$AB=2,BC=1,\angle ABC=60°$.点$E$和点$F$分别在线段$BC$和$CD$上，且$\overrightarrow{BE}=\frac{2}{3}\overrightarrow{BC},\overrightarrow{DF}=\frac{1}{6}\overrightarrow{DC}$，则$\overrightarrow{AE}·\overrightarrow{AF}$的值为____ 。