39.Why is PM2.5 the most dangerous to human's health?
A. Because it is a kind of "fine" particle.
Because it is of very small size.
C. Because it is only one-thirtieth the average width of a human hair.
D. Because it can go into and stick to human's lungs.
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40.How can we reduce the amount of PM2.5?
A. Use chemicals to clean the air.
B. Decrease all kinds of burnings..
C. Improve crushing or grinding operations.
D. Stop industrial processes
"应用乔木、灌木、藤本及草本植物来创造景观，充分发挥植物本身形体、线条、色彩等自然美，配植成一幅幅美丽动人的画面，供人们观赏", 被称为：（）
中国古典园林的三大源头分别是：（ ）、（ ）、（ ）。
以下哪个时期是中国古典园林发展的兴盛期？
A. 秦、汉时期
B. 唐宋时期
C. 魏晋南北朝
D. 元明清时期
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