阅读理解。
Trees are useful to man in three important ways: they give him wood and other useful things, they give him
cool places, and they help to stop drought and flood.
Unluckily, in many parts of the world, man has not found that the third of these points is the most important.
Man wants to make money from trees,so he has cut them down in large numbers, only to find that without
them he has lost the best friends he had. And also, he is usually too careless to plant and look after new trees.
So the forests slowly disappear.
This does not only mean that man will have fewer trees. The results are even worse: for where there are
trees, their roots break up soil-make the rain in-and also bind the soil, thus stopping it from being washed away
easily; but where there are no trees, the rain falls on hard ground and flows away, causing floods and carrying
away the rich top-soil. When all the top-soil is gone, nothing is left but useless desert.
1. The most important points of trees to man is _____. [ ]
A. they help him to make money
B. they give him cool
C. they give him wood and other things
D. they help him to stop drought and floods
2. In many places forests slowly disappear because _____. [ ]
A. many trees have been cut down by man
B. new trees are not well looked after
C. man has not paid enough attention to planting trees
D. all the above
3. Land becomes desert after all trees are cut down because _____. [ ]
A. roots of trees break up the soil
B. there are too many rainfalls
C. strong winds bring a lot of sand
D. there are no longer trees to keep the rain and protect the top-soil.
4. Which title best fits the passage? [ ]
A. Trees and Man B.The Function of Tree Wood
C. How do People deal with Trees?
D. The Usage of Tree Roots
56×5角钢(A=2×541.5=1083mm2，imin=21.7mm)；下柱支撑设置一道，其斜杆的长度l1=9.12m，采用2[8槽钢(A=2×1024=2048mm4，imin=31.5mm)。柱的截面宽度b=400mm。钢材用Q235钢。水平地震作用标准值如图(b)所示。取γRE=0.8。
