与其他动物相比，哺乳动物后代成活率较高的主要原因是（    ）

    My name is Amy . It is spring now . But my favourite season is summer . I often wear my skirts and T-shirts . Sometimes I wear my jeans . In summer it is often sunny and hot . Sometimes it's rainy in summer . I like swimming , so I usually swim with my friends on weekends in summer . In summer , I like eating ice-creams . They are delicious . I like summer so much . It brings so many wonderful things to me .

 初二（4）班的陈强学习成绩中等，平时一贯好动。一次上语文课时与同桌打闹，被张老师看到，随即批评他上课不专心听讲，扰乱课堂秩序。陈强非常不服气，因为这件事确实是由同桌先挑起的，而张老师没有批评同桌，只批评了他一个人。陈强觉得张老师对人不公，偏爱那些听话、成绩好的同学。但陈强当时和事后也没有向张老师多作解释，只是心中不满。以后，陈强上课越来越不听讲，和同学讲话，乱动同学的东西，不仅师生关系紧张，而且成绩也越来越糟糕。

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