Here is an astonishing and signficant fact:Mental work alone can't make us tired. It sounds absurd. But a few years ago, scientists tried to find out how long the human could labor without reaching a stage of fatigue(疲劳). To the amazement of these scientists, they discovered thett blood passing through the brain, when it is active, shows no fatigue at all! If we took a drop of blood from a day labourer, we could find it full of fatigue toxins(毒素) and fatigue products. But if we took blood from the brain of Albert Einstein, it would show no fatigue toxing at the end of the day.

    So far as the brain is concerned, it can work as well and swiftly at the end of eight or even twelve hours f efforts as at the beginning. The brain is totally tireless. So what makes us tired.

    Some scientists declare that most of our fatigue come from our mental and emotional(情感的) attitudes. One of England's most outstanding scientists. J. A. Hadfield,says,“The greater part of the fatigue from which we suffer is of mental origin. In fact,fatigue of purely physical origin is rare.” Dr. Brill, a famous American scientist, goes even further. He declares,“One hundred percent of the fatigue of a sitting worker in good health is due to emotional problems.”

    What kinds of emotions make sitting workers tired?Joy?Satifaction?No!A feeling of being bored,anger,anxiety,tenseness,worry,a feeling of nt being appreciated---those are emotions that tire sitting workers.Hard work by itself seldom causes fatigue.We get tired because our emotions produce nervousness in the body.

下表是对部分犯罪少年过往行为的调查数据，它告诉我们（   ）

①不良行为必然发展为违法行为                 ②不良行为任其发展，可能导致违法行为

③犯罪少年都有打架和旷课行为                 ④青少年要避免不良行为，预防违法犯罪

“美丽中国”的提出，意味着我们不再是不惜一切代价地谋求发展，而是开始驻足审视；意味着我们不仅要金山银山，更要绿水青山；意味着我们“把生态文明建设放在突出地位，融入经济建设、政治建设、文化建设、社会建设各方面和全过程”。

染色体是由____和____两种物质组成的．____是遗传物质，因此可以说染色体就是遗传物质的载体．