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试解释为什么聚氯乙烯在200oC以上加工会使产品颜色变深？为什么聚丙烯腈不能采用熔融纺丝而只能采用溶液纺丝？

答案

参考答案：聚氯乙烯加入到200℃以上会发生分子内和分子间脱去HCl反应，使主链部分带有共轭双键结构而使颜色变深。聚丙烯腈在高温条件下会发生环化反应而不会熔融，所以只能采用溶液纺丝。

填空题



In the early 1970’s, Citibank of New York City became one of the first financial institutions to install ATM’s on a large scale. Since then, the (1) of automated teller machines has increased dramatically. At the time, the bank hoped (2) their operating costs by replacing human tellers with machines. Little did they (3) that the cash machines would become so successful. However, as more banks added (4) automated service through ATM’s, the machines mushroomed all over the world.
To use an ATM, all you need is a (5) issued by your bank. Your bank may also (6) through a credit card, such as MasterCard or Visa. To begin a (7) , you need to insert the card into an ATM and punch in a personal identification number on the (8) . The personal ID number may consist of (9) and can prevent anyone from using the card.
The ATM next flashes instructions on its (10) for carrying out transaction. To get cash, for example, you are instructed to (11) that indicate whether the money should be withdrawn from a checking or a savings account and the (12) . This request is then displayed on the screen. After you press a button to (13) that the information is correct, the ATM goes to (14) .
How safe, you may ask, is banking by ATM The (15) is meant to prevent anyone, no matter who you are, from using a cash card (16) . If you enter the wrong ID number for a card, a message on the screen will (17) . If you fail to produce the correct number for three times, most machines will (18) .As another precaution against (19) , the bank generally limits the amount that may be withdrawn by cash card (20) , say, to $ 200.
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单项选择题

步行周期的支撑相占步行周期的（）

A.20％

B.30％

C.40％

D.50％

E.60％
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