某小区有1000户，各户每月的用电量近似服从正态分布N（300，l00），则用电量在320度以上的户数估计约为（   ）

青藏牧区所在的地形是（   ）

2009年11月19日，美国驻华使馆教育交流中心主任蓝莺女士表示。“2008～2009学年，中国赴美国留学的人数已接近10万，比前一学年增长21％。”材料反映了
①教育是文化传播的重要途径 ②人口迁徙是文化交流的重要途径之一 ③我国尊重文化多样性，促进文化交流 ④我国推动中华文化走向世界

     All students need to have good study habits. When you have good study habits, you will learn things quickly and also remember them easily. Do you like to study in the living room? This is not a good place, because it is usually too noisy. You need to study in a quiet place, like your bedroom A quiet place will help you only to think about one thing. When you study, do not think about other things at the same time. Only think about your homework. If you do so, you will do your homework more quickly, and you will make fewer mistakes.
    Good study habits are very important. If you do not have them, try to learn them. If your studyhabitsare already good. Try to make them better.

设等差数列$\left\{a_n\right\}$的前$n$项和为$S_n$,若$a_1=9$，$a_6+a_4=2$, 则当$S_n$取最大值$n$等于（    ）