人们把花的特征作为植物分类的重要依据的原因是

    Now a hot topic____(discuss) about whether the old computers in our school should be replaced____new ones. With the rapid____ ( develop) of science and technology, there is no doubt ____our old computers are totally out of date. The headmaster as well as the teachers and students____(be) not satisfied with the slow working speed of the computers. Therefore, all of them request to have a new computer so that their problems can be solved ____ （success).____the school should consider is ____to deal with the old computers wisely. Personally speaking,____(throw)  them away to the place____they are regarded as rubbish is not a good idea.

如图，已知A，B，C，D是同一直线上的四点，看图填空：AC=____+BC，BD=AD﹣____，AC＜____．

请用线将各种生态系统及对应的特点连起来．

A、草原生态系统   ____                              a、多水和过湿的条件下形成

B、湿地生态系统   ____                              b、分布在较湿润地区，动植物种类繁多

C、森林生态系统   ____                              c、分布在干旱地区，年降雨量少，缺少高大植物

D、农田生态系统    ____                             d、人工生态系统，以农作物为主体

E、城市生态系统      ____                           e、以人为主体，与自然环境、社会环境相互作用的统一体