根据短文内容及首字母提示完成短文。（每空一词）
   Friends play an important part in our lives. We may have many friends, but we often don't understand c____how we make friends. We may get on well with a n____of people, but we usually make friends with a very few of them.
   People use the word "friend" freely. Friends may k____each other since childhood or they may recently meet. In fact, people many have different k____of friends such as school friends, work friends, or sports friends. A person may c____different friends for different activities. If you play with s____  on the playground, the person is your playmate. If you often play chess w____ a friend, the person is your chess partner.
   One may have many good friends and only one best friend. " Best friends" or "close friends" are u____two people who have known each other for a long time. A close friend is someone who is able to s____ experience, joys and sorrows(悲伤) with you. This is a person you can rely on (信赖) when you are in n____, and this is the person to whom you are willing to lend a helping hand.
   Friendship is one of life’s greatest treasure.

光污染已成为21世纪人们关注的问题。据测定，室内洁白、平滑的墙壁能将照射在墙壁上的太阳光的80%反射，长时间在这样刺眼的环境中看书学习会感到很不舒服。如果将墙壁做成凹凸不平的面，其作用之一可以使照射到墙壁上的太阳光变成散射光，达到保护视力的目的，这是利用了光的（    ）

某几何体的三视图如下图所示，它的体积为(    )

对中国人而言，19世纪是一个危机的世纪，一个艰难的世纪，一个抗争的世纪。在这个世纪中，最先强迫中国签订近代第一个不平等条约的国家是（   ）

某市按政策规定对报废老旧汽车、“黄标车（注：指污染物排放达不到国Ⅰ标准的汽油车和达不到国Ⅲ标准的柴油车）”并换购新车给予3000~6000元的补贴。至2009年11月12日，共销售家电以旧换新产品31000余件，回收旧家电32000余件，补贴700余万元。下列说法正确的是    ①回收的旧家电和旧汽车仍是商品，因为国家财政补贴回购②回收的旧家电和旧汽车仍是商品，因为它们还在进行商品交换③以旧换新活动有利于鼓励人们遵循保护环境、绿色消费原则④以旧换新活动有利于拉动消费需求，缩小居民收入差距