下列原始人类出现的先后顺序是（　　）

①半坡遗址   ②元谋人   ③北京人   ④河姆渡遗址

阅读理解

    As we know, when the air warms up and rises, it has water vapor(水蒸气) in it. When the vapor hits particles of dust(尘埃粒子) in the air, it forms droplets(液滴). When these droplets become big and heavy, they fall to the earth as rain. Recently, Israeli scientists have discovered that air pollution may stop rain from falling.

    They found the smoke from cars and factories adds to the particles in the air. Because there are so many of these particles, more small droplets form, but they are not big enough to fall down as rain. These clouds are called dry clouds.

    The only way to get back rain is by cleaning up the environment. Or a time may come when clouds will form but no rain will fall.