国共关系直接影响着中国革命的前途和命运，下面两党关系演变中，实现再合作的原因是（    ）

阅读下列短文，从每小题的三个选项中选出最佳选项。B
    Water comes from the sea, the rain and the tap. Animals, plants and human beings all need water. It is important for our daily (日常的）life. We use water to wash clothes and vegetables. We use water to cook food, make drinks, wash our face and so on. Firemen use water to put out fires. Fanners use water to grow crops. Water is useful. We mustn't waste (浪费）water. We must save water. Turn off the tap when you are brushing your teeth. Don't take a shower too long. Use the same water many times.

近年来，雾霾天气频现，空气质量受了社会的广泛关注，《苏州市市区禁止燃放烟花爆竹规定》已于2017年1月1日起施行。本规定适用于吴江区、吴中区、相城区、姑苏区、苏州工业园区、苏州高新区（虎丘区）等行政区域，目的在于防止环境污染，坚持绿色发展，推进美丽中国建设。

填上适当的整式，使等式成立：（x﹣y）²﹣____ =（x+y）²．

关于RNA的叙述，错误的是（　　）