If you're looking for a reason to care about tree loss,this summer's record-breaking heat waves might be it.Trees can lower summer daytime temperatures by as much as 10 degrees Fahrenheit,according to a recent study.But tree cover in US cities is shrinking（缩减）.A study published last year by the US Forest Service found that we lost 36 million trees annually from urban and rural communities over a five-year period. "If we continue on this path,cities will become warmer,more polluted and generally unhealthier for inhabitants," said David Nowak,a senior US Forest Service scientist and co-author of the study.Nowak says there are many reasons why our tree cover is declining,including hurricanes,tornadoes,fires,insects and disease.But the one reason for tree loss that humans can control is sensible development.
Nowak says there is a downside to trees too,such as pollen allergy（花粉过敏）or large tailing branches in storms,and people don't like sweeping leaves.But,he says,there are ways cities and counties can manage trees to help communities thrive（繁荣）.Urban forests especially need our help to replace fallen trees.Unlike rural areas,it is very difficult for trees to repopulate themselves in a city environment with so much pavement and asphalt（沥青）."A lot of our native trees can't actually find a place to drop a seed so they can regenerate,explains Greg Levine,co-executive director tor Trees Atlanta. " That's why the community has to go in and actually plant a tree because the areas just aren't natural anymore.
Nowak says the first step is caring for the trees on your own property（财产）."We think we pay for our house,and so we must maintain it.But because we don't pay for nature,we don't need to.And that's not necessarily true.



Why does the author mention "trees can lower summer daytime temperatures"？ ______

A. To tell the temperatures in summer are high.
B. To explain the reason of tree loss.
C. To tell trees are helpful.
D. To introduce the topic.

How can humans control tree loss according to Nowak？ ______

A. Develop cities in reasonable ways.
B. Prevent fires form damaging trees.
C. Improve climate to let trees grow.
D. Decrease insects in cities.

Why do forests in cities need our help？ ______

A. Because trees don't grow in a city environment.
B. Because native trees don't drop seeds any more.
C. Because trees in urban areas can't regenerate naturally.
D. Because humans want to plant more trees.

What is the purpose of this passage？ ______

A. Describe the importance of trees in cities.
B. Appeal people to protect trees in their surroundings.
C. Ask people to plant trees with the author.
D. Show the number of trees in the US is declining.