某小组同学在讨论、解决大气压的问题时，就公式p=F/S和p=ρgh的运用做了如下思考：
① 他们认为：公式p=F/S是压强定义式，具有普适性。根据该观点，若已知地面附近大气压为p₀，地球表面积为S₀，请你估计地球表面大气的总质量为____ 。
② 他们认为：气体和液体有相似的特点，同样大气压跟液体内部的压强也有相似之处。因此在某些情况下，可以利用公式p=ρgh来讨论大气压强的问题。
(a) 请你举例说明“跟液体内部压强相似的大气压的特点”。____ 
(b) 查阅资料知道：空气在海拨2千米以内可近似认为是均匀的，其密度为ρ₀。若静安寺附近某幢大楼的高为H，则该大楼的楼顶与地面间大气压强的差为____ 。

某地区有7部电话，要使这7部电话都能互相通话，如果使用交换机需要多少对电话线（　　）

下列四幅照片反映的是第二次世界大战中的一些历史事件，其中与美国有关的是(    )

若直线y=kx+1与圆x²+y²=1相交于P、Q两点，且∠POQ=120°（其中O为原点），则k的值为（　　）

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.