题目

单词拼写(共10小题;每小题1分,满分10分) I have no idea of the ___________(平均) size of classes in the UK. Great changes have taken place in our country ___________(近来). Good ________(准备) should be made before we have the important meeting. Don’t feel e__________ about being different from others. I would p________ playing outdoors to watching TV at home. __________ (注意力集中) more on your work or you will be fired. I found it difficult to keep my b________ on the icy road. She performed the most c_________ task without a mistake. I’m learning German, but I sti小刚为书房买灯，现有两种灯可供选择，其中一种是9瓦（即0.009千瓦）的节能灯，售价49元/盏；另一种是40瓦（即0.04千瓦）的白炽灯，售价18元/盏，假设两种灯的照明亮度一样，使用寿命都可以达到2800小时，并已知小明家所在地的电价是每千瓦时0.5元。（1）小刚想在这两种灯中选购一盏①当照明时间是多少小时使用两种灯的费用一样多？②直接指出，当照明时间在什么范围内，选用白炽灯费用低？当照明时间在什么范围内，选用节能灯费用低？（2）小刚想在这两种灯中选购两盏，假定灯的使用寿命都是2800小时，而计划照明3000小时，请你帮他设计一种费用最低的选灯方案，并说明理由。