答案　A　

　　．If a human being is hit by nuclear radiation, he may ________．

　　A．die of cancer after many years

　　B．die immediately

　　C．have a child who may be born weak

　　D．all of the above

　　答案　D　(2015·山东省威海市二模)

　　Run, swim, lift weights, do yoga—do something, because it's good for you. Many studies tell us that regular exercise not only helps our bodies but improves memory, and eases anxiety. But what is happening to the mind during workout to create all these benefits?

　　A new study from Sweden suggests that a specific protein found in trained muscles might contribute to exercise's mental benefits. This protein, the researchers found, might help our bodies process molecules that, if out of balance, can stress us and contribute to depression(萎靡不振). The findings were published in the journal Cell.

　　To test this, the researchers kept attacking some mice with flashing lights, loud noises and interrupting their sleep for weeks. Predictably, this treatment makes the mice show signs and biochemical markers of depression.

　　But not all the mice showed those signs. One group, specially raised to have high levels of a protein called PGC­1alpha1 (the one in trained muscles), did not show those depression signs. PGC­1alpha1 increases production of an enzyme that breaks down a chemical called kynurenine. Kynurenine helps the body respond to an activated immune system(活性免疫系统), but it also plays a role in depression.

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