题干

以下有关遗传变异的说法正确的是(    )

A:三倍体无子西瓜不育,其变异也不能遗传给后代

B:取二倍体植株花粉,进行花药离体培养获得的皆为纯合植株,自交无性状分离

C:在镜检某基因型为AaBb的父本细胞时,发现其基因型变为AaB,该变异为基因突变

D:在有丝分裂和减数分裂的过程中,会由于非同源染色体之间交换一部分片段,导致染色体结构变异

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答案(点此获取答案解析)

D

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    Cycling or even walking in city areas can be a little dangerous, thanks to the fact that one is sharing the road with vehicles that are increasingly getting larger and heavier. A recent study proves that of the 1.27million people that die in road traffic crashes each year, about half are walkers, motorcyclists and cyclists. Now there may be a solution that could provide at least some help to protect this helpless group---an airbag!

    If you are worried that this safety measure will involve you taking around a heavy package or worse still, wrapping yourself inside an ugly plastic bag, the bag is fitted not on the human, but on the outside of the vehicle. And, while there are several versions of the idea in the works, the one most recently unveiled by Dutch car company TNO, seems to be the most advanced and ready to go into production.

    In the works since 2011, the airbag covers only the lower part of the windshield. This will provide the much needed buffer(缓冲)between the person's head and the pane of glass he/she would otherwise meet.

    The chain of events leading to an airbag cause are quite simple---A camera fitted beneath the rear-view mirror monitors the vehicle's closeness to a walker or cyclist. Any contact with either one of them sets off the sensors in the car's bumper and quickly blows up the airbag. In addition to that the sensors also set off the car's automatic brake, reducing the chances if an even worse injury.

    In tests conducted using a model, the success of not getting injured after being hit by a car travelling at about 40km/h was about 50-50! While not perfect, it will still result in reducing the number of deaths by a huge amount.

    With TNO ready to license its technology to car makers and many more companies trying to develop similar concepts, we would not be surprised if outer airbags become a standard feature in every car pretty soon.