D. Put shelves at the back of the classroom for students to leave their bags.
(D)
(2017·广州模拟)
We each have a unique genetic(基因的, 遗传的)make-up. Every cell of our body has the same set of about 100, 000 separate genes made of DNA. These are the instructions for producing a person. Genes decide everything from the colour of our skin to the way our brain works. We have one of several billion combinations of DNA which come from the random mixing of our parents’ genes. Except for identical twins(同卵双胞胎), no one has the same combination as another person. We are unique! We are unique in another way, too:in the way we are raised and all the experiences we have from before birth to adult life. These experiences influence us, our behaviour and attitudes, and the choices we make.
But are genes or life experiences more important in shaping our appearance and personality? Scientists are studying twins to find out. One set of twins occurs every 70 births—some are identical and others are non-identical twins. Identical twins are special because they share exactly the same genes and often the same environment. Non-identical twins are more like ordinary brothers and sisters.
Some identical twins have been adopted and brought up in different homes. With identical genes but a different home environment, scientists can study twins to see how much a particular feature depends on the genes we inherit(继承). For example, we know that eye problems, like short-sightedness, are mostly genetic. But resistance to pain is largely dependent on experiences. Genes also influence our eating habits. Identical twins brought up apart often like to eat at the same time of day and feel full after eating the same amount. Non-identical twins in similar circumstances have more varied eating habits. Identical twins are also more likely to follow the same patterns for marriage and divorce than non-identical twins.
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