1)在开头出现;
2)在结尾出现;
3)在开头和结尾同时出现，首尾呼应;
4)有些文章的中心思想贯穿全文，并没有用一句话明确表达出来，这就要求学生学会归纳概括。
(三)实战演练
4.Excused from recycling because you live in a high rise with a rubbish chute (垃圾道)? You won't be for long. Miami's Mark Shantzis has made it simple for those living in tall buildings to use the chute and cycle, too.
In Shantzis' Hi-Rise Recycling system, a chute leads to a pip-shaped container with six boxes that can turn around when operated. The system, which fits in the same space as the chute and container now in use, enables ,glass, plastics, paper, metal, and other rubbish to go into separate boxes.
The system is controlled from a board foxed next to the chute door. The board has a button for each class of recycling materials. At the press of a button, a microcomputer locks all other floors' chute doors and sets the recycling container turning until the right box comes under the chute. The computer also counts the loads and gives a signal by phone when the box is full. And a particular piece of equipment breaks up the non-recyclables.
Sorting(分类) recyclables before they are collected saves the use of expensive materials recovery equipment which otherwise has to do the sorting. Such equipment often makes recycled materials very expensive, so expensive that tones of recyclables remain wasted. Shantzis believes his system could help recycled materials become more cost-effective.
The purpose in writing this text is _________.
A to encourage people to recycle their rubbish.
B to introduce a recycling system for high rises
C to describe the use of computer technology in recycling
D to explain the need for rubbish collection in high rise