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问题 选择题

下列实际应用中，主要从密度角度考虑的是（　　）

A．用塑料做砂锅的手柄

B．用厚钢板做潜水艇的外壳

C．用铜丝做导线

D．用塑料泡沫做电影场景中倒塌的“墙壁”

答案

A、用塑料做炒锅的手柄是为了防止炒锅使用时烫手，利用了塑料的导热性差，不合题意；

B、用厚钢板做潜水艇的外壳，是因为液体压强随深度的增加而增大，因此是利用了钢板的硬度大，不合题意；

C、用铜丝做导线主要利用了铜具有良好的导电性，电阻小，不合题意；

D、塑料泡沫密度较小，同体积时与其他材料相比，质量较小，因此，在表演场景中制作成倒塌的“墙壁”，可保护演员的安全，符合题意；

故选D．

选择题

(附加题)PCR是一种体外迅速扩增DNA片段的技术，下列有关PCR过程的叙述，不正确的是[ ]

A．变性过程中破坏的是DNA分子内碱基对之间的氢键

B．复性过程中引物与DNA模板链的结合是依靠碱基互补配对原则完成的

C．延伸过程中需要DNA聚合酶、ATP和四种核糖核苷酸

D．PCR与细胞内DNA复制相比所需要酶的最适温度较高
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单项选择题

Questions 16～20


Napoleon famously described the British as a nation of shopkeepers. These clays it would be equally true to describe them as a nation of shoppers. Either way, London is the UK’s shopping mecca; if you can’t find it here you probably can’t find it at all.
Some London shops are more or less tourist attractions in their own right. Few visitors come away without popping into Harrods. This famous store is a real one-off. The toilets are lab, the food hall enough to make you swoon, and if they haven’t got what you want, it probably doesn’t exist. No other store has such a sense of sheer, outrageous abundance. Since Absolutely Fabulous brought Edina and Patsy steaming onto our screens, Harvey Nichols ("Harvey Nicks") has become another must-see attraction. It boasts a great food hall, an extravagant perfume department and jewellery to save up for. But with all the big names from Miyake to Lauren, Hammett to Calvin Klein and a whole floor of up-to-the-minute menswear, it’s fashion that Harvey Nichols does better than the rest. The selection is unrivalled and the prices high, although the sales offer some great bargains, and the store’s own clothing line is reasonable.
Carnahy Street still reeks of the 60s although it’s had something of a revival since the "Cool Britannia" kick brought Union Jack dresses back into fashion. The last punks have long since slunk away from Chelsea’s King’s Rd but there are still plenty of interesting shops slotted in amid the high-street chains.
Oxford St and classier Regent St come into their own in the six weeks running up to Christmas when they’re festooned with lights. At other times of the year Oxford St can be a great disappointment. Selfridge’s is up there with Harrods as a place to visit and the farther east you go the tackier and less interesting it becomes.
Although most things can be bought in most parts of town, there are also streets with their own specialties; Tottenham Court Rd, for example, is one long electrical goods shop (watch out for rip-offs though), while Charing Cross Rd is still the place to come for offbeat books.
Many tourist attractions have excellent shops selling good-quality souvenirs like mugs, pens, pencils, stationery and T-shirts, often with themes to match their content (war books and videos at the Imperial War Museum). By buying from these shops you help contribute towards the building’s maintenance, especially important in the cases of those without entry charges.

The word "revival" (para. 3) means ______.

A. cooling off
B. holding sway
C. coming back
D. firing up
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