题目

2．PM2.5是指大气中直径小于或等于2.5微米的颗粒物，2.5微米等于0.0000025米，把数字0.0000025用科学记数法表示为（ ）A．2.5×106B．2.5×10-6C．2.5×10-7D．25×10-5 答案：分析 绝对值小于1的正数也可以利用科学记数法表示，一般形式为a×10-n，与较大数的科学记数法不同的是其所使用的是负指数幂，指数由原数左边起第一个不为零的数字前面的0的个数所决定．解答 解：把数字0.0000025用科学记数法表示为2.5×10-6，故选：B．点评 本题考查用科学记数法表示较小的数，一般Sixteen is a special age in the United States.There’s often a special celebration for a girl’s 16th birthday---a big party called Sweet-16. It is like a wedding reception(结婚晚宴) in the number of guests and gifts handed out.About a month before the party, the girl’s family sends out invitations to the guests. They must buy a special present for the birthday girl and the girl guests find good dresses to wear. A sweet-16 present usually costs $40 to $60. This is about $20 more than the usual-cost of a birthday present.The party itself is not held at someone’s house. The hostess rents(租赁) a reception hall for the event. Food, cake and a DJ are provided. Around 6’clock, guests arrive and meet the birthday girl. Then the girl and her father have a special father-daughter dance. After that, guests dance for about 30 to 45 minutes.Then there is the most important part of the Sweet-16 party, the candle-lighting ceremony.The sweet-16 hostess stands behind 16 unlit(未点燃的) candles and dedicates(献给) each candle to her friends, family, and anything that she feels is important to her. A lot of crying and hugging goes on at this time.Afterwards, people sit and eat. Most guests begin leaving around 11 o’clock.This school year, I have been to two Sweet-16s. I am one of the youngest students in my grade (My birthday is in December). I’ll have to wait another year before I can have my own special Sweet-16 party. I can’t wait for it!1.Why is sixteen a special age for an American girl?  2.How much does a usual birthday present for a teenager cost in the USA? 3.What is the most important part of the Sweet-16 party? 4.How old is the writer? 5.What do you think of the Sweet-16 party in the USA? (请自拟一句话作答)