题目

    Birds in cities are damaging their health by trying to sing above the noise of urban life．    New research shows that male birds are trying to compete against traffic and city sounds．They are now singing louder and at higher frequencies，which could harm their vocal cords(声带)．Some birds are choosing to sing at night instead of during the day．This makes them more open to attack and also creates stress and exhaustion．   “The difference between urban and rural birdsong is becoming so big that the two groups could now be unable to communicate．This could lead to inbre 汉代晁错在他的《论贵粟疏》中写道：“勤苦如此，尚复被水旱之灾，急政暴赋，赋敛不时，朝令而暮当具。有者半贾而卖，亡者取倍称之息。于是，有卖田宅者、鬻子孙以偿责（债）者。”材料反映出小农经济的特点是（　） A．男耕女织     B．自给自足     C．脆弱性         D．连续性