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为什么有的聚合物热稳定性好，而有的热稳定性差？举例说明。

答案

参考答案：

有的聚合物热稳定性差的原因是大分子链中含有叔碳原子或季碳原子，而凡是和叔碳原子或季碳原子相邻的链都是不稳定键，受热时容易断裂和降解。不含叔碳原子或季碳原子的聚合物热稳定性好些。如，聚丙烯主链上含有碳叔原子，抗降解的稳定性比没有碳原子的聚乙烯差。

多项选择题

一般纳税人生产销售下列货物，可按简易办法依照6%征收率计算缴纳增值税的是（）。

A.县以下小型水力发电单位生产的电力

B.寄售商店代销寄售物品

C.以水泥为原料生产的商品混凝土

D.建筑用的砂、土、石料
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问答题

The teaching of English as a second language (ESL) in schools has had a history of conflicting arguments, interesting innovations and some very positive methodological changes. To understand the present situation, it is necessary to consider the past and the wider educational context which has a bearing on it.

Until quite recently, approaches to ESL work have been ply influenced by methods developed to teach English as a foreign language to older learners. These methods placed much emphasis on drills, exercises and remedial programs that focus on language in abstraction. 46) The prescriptive nature of such methods and the demands they made on the teacher’s time developed the belief that ESL work could be tackled only by the specialist ESL teacher working with small groups of children. Such an approach does not fit comfortably into current notions of learning and teaching in the primary school, nor does it sufficiently equip ESL learners in the secondary school to benefit from normal schooling. 47) In prescribing what language is to be taught, it has ignored what children bring to the learning task and the choices they make about how and what they want to learn. Furthermore, the location and organization of language provision did not measure up to the demand. 48) The language centers and English language services all contributed to providing special and concentrated teaching of English as a second language in small groups, varying in size from four or five to fifteen. Whatever the pattern of provision, the main aim was to give pupils sufficient English to enable them to join normal schools as quickly as possible. The success of such special provision depended very much on the close and constant liaison of language teachers with the subject teachers and the class teachers and on the continuity of learning experiences provided by them. 49) One of the important disadvantages of language centers and withdrawal groups was that ESL children were being taught away from those English- speakers who provide the most powerful models, i. e. their peer group. Peer-group interaction is an important element in any learning situation, but its particular strengths in a classroom with ESL learners cannot be overemphasized.

50) The separation of second language learners from the mainstream classroom cannot easily be justified on educational grounds, since in practice it leads to both their curriculum and language learning being impoverished.

48) The language centers and English language services all contributed to providing special and concentrated teaching of English as a second language in small groups, varying in size from four or five to fifteen.
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