问题
选择题
用平面去截下列几何体，不能截出三角形的是（ ）
A．长方体
B．三棱锥
C．圆柱
D．圆锥
答案
正方体沿体面对角线截几何体可以截出三角形，三棱锥沿顶点截几何体可以截得三角形，圆柱不能截出三角形，圆柱沿顶点可以截出三角形，故选C．
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用平面去截下列几何体，不能截出三角形的是（ ）
A．长方体
B．三棱锥
C．圆柱
D．圆锥
正方体沿体面对角线截几何体可以截出三角形，三棱锥沿顶点截几何体可以截得三角形，圆柱不能截出三角形，圆柱沿顶点可以截出三角形，故选C．
Passage Two
| Running a School Book Stall 开办学校书店 I assume that the desirability of a school book stall needs no urging. Many schools sell food and toys. If we do not sell books it is surely strange Many schools serve areas where book shops do not exist and the only books brought before children for buying are the dubious selections of supermarkets. Moreover even in communities where a good book shop is available the guidance which can be given at the book stall is valuable, as we soon found. Essentially the school book stall is an extension of the encouragement and guidance in private reading which is part of the work of the English teacher. The first essential then, in setting up shop is a teacher particularly interested in children reading and in building up as wide as possible a knowledge of books to suit the school’s range of pupils. Given the teacher, the next requirement is a bookseller willing to supply you. In some cases you will be able to obtain your books on credit, paying as you sell, but if the school can find a sum to purchase its stock, or at least a part of it, this is a great help. Having found your supplier you then approach the Publisher’s Association for a Book Agent’s licence. The licence entitles you to a discount on your purchase through your chosen supplier, the usual discount being 10% with service. Service usually consists of delivery and a sale or return arrangement, the latter essential in allowing you to be enterprising and experimental in your stock. Without service a slightly higher discount is given but the former arrangement is clearly preferable. The biggest, indeed the only considerable, cost in running the book stall is the occasional theft of a book and this may well vary from school to school but the presence of the teacher and the alertness of the assistants is largely deterrent, and the discount should cover this and any other smaller expenses. Browsing is essential. The books must be handled. You cannot keep them safe and immaculate behind glass. For equipment the only essentials are some tables on which to display the books and a cupboard to store them in. Incidentally an arrangement of books with covers rather than spines visible seems to be vastly more attractive and accessible to children who have not the habit of browsing. A single way out past the cash desk is helpful to security and we record details of each purchase including the age of the buyer both for reordering and as interesting information on reading habits. Initially we stocked two hundred titles and the selection has grown to close on a thousand. It is convenient if cash or credit allows you to have duplicate copies of popular titles. What is stocked must depend on the teacher in charge. What you are prepared to sell in the cause of encouraging interest in reading will obviously be an individual judgment. Sales for their own sake are in the school context obviously purposeful and the teacher needs to be able to explain to interested parents why he thought a given book valuable for a certain child. There are always more offers of help from pupils than we can accept. The assistants serve, recommend, order, make posters and arrange displays. Some of the least able pupils have worked devotedly at the book stall. Publicity is vital. We have two display cases on the school approach containing forty books changed fortnightly and they arouse a lot of interest. Teachers’ recommendations, book lists, beginnings of stories read to classes, do much. Some classes buy a book a week between them. The book stall is always open on such occasions as Parent’s Evenings. We open twice a week in the lunch hour and we sell twenty to forty books a week, commercially not much but in our opinion well worth the effort. |
To what does "this may well vary" refer
A．The expenditure on books
B．The num.ber of books stolen
C．The vigilance of the assistants
D．The number of books stocked