太阳能是大自然赋予人类的一种取之不尽、用之不竭的能源．在太阳进行着大规模的核聚变，其释放的能量主要以电磁波的形式辐射出来．地球每年所接受的太阳能有6×10¹⁷kW•h．

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A

When I drove to work yesterday, I met a traffic jam（交通阻塞）. As I looked out of my window, a young man was walking on the street. He was walking to the bridge. There were some homeless men with their cups around there. The young man went to one of them. He brought out some money, said a few words to him and put the money into his cup. He continued（继续） to walk and then saw another homeless man. He stopped for a minute, said a few words and then gave the man a candy.

And then he saw the next homeless man. He was shivering（冷得发抖） because of coldness. The young man now stopped and gave his scarf to him. He then said goodbye to him with a big smile and went away in the snow.

At that moment, I was in tears（流泪）. I gave this young man my best wishes with love and blessings.

①平行四边形的对角线互相平分；

②对角线互相平分的四边形是平行四边形；

③菱形的对角线互相垂直；

④对角线互相垂直的四边形是菱形．

其中真命题的个数为（   ）

始祖鸟的大小和乌鸦差不多，图为始祖鸟化石，从化石分析，始祖鸟的身体被覆____；具有和鸟翅膀一样的____，但前肢的末端有指，指的末端有____；它的嘴里有____，嘴的外形像____，尾长并有尾椎骨．也就是说，始祖鸟的身体结构既和____有相似之处，又和____有相似之处．

过去常用水作冷却剂给汽车的发动机降温，这是利用了水的比热容较 ____的性质，某汽车水箱中装了5kg的水，发动机运行了一段时间后，水温升高了20℃，则在这个过程中，水吸收的热量是 ____J．水的比热容为4.2×10³J/（kg•℃）．