下图为“2005～2013年黄河下游某处河床高出两岸地面高度的变化图”。引起这种变化的外力作用主要是（  ）

下列物质由离子构成的是（）

任务型阅读
    There are some easy things you can do to protect the environment and the earth. Choose ideas from the list or come up with a few of your own.
    Plant flowers, grass or trees.
    Whenever you visit a park or a beach, take away what you take there—keep rubbish in a bag until you can put it in a bin.
    Turn off the lights and TV sets when you leave the room. This can save a lot of electricity.
    Turn off the tap （水龙头）when you brush your teeth. You can save some water by not letting it run. Also, use a glass cup instead of a paper cup because this saves paper.
    Keep the doors and windows closed in winter to keep cold air out.
    Give your old clothes to poor chirldren instead of throwing them away.
    Use both sides of the paper.
    Stop pouring dirty water into the rivers or lakes nearby.
Encourage all your friends to do the same things as you do to help protect the earth. You don't have to wait until Earth Day to do these things. Make every day Earth Day. If everyone makes a contribution(贡献) to protecting the environment, the world will become much more beautiful.

What ________great fun it is to have a swim on a hot day in summer!

导致经济重心南移的最具有决定性意义的因素是