The ozone layer traps heat from the sun and leads to the rise of the earth's surface temperatures.
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Tourism has little impact on natural resources and the environment.
A. 对
B. 错
People are supposed to spend quite a lot of money from tourism proetecting the environment.
A. 对
B. 错
Because of its damage to the environment, tourism will cease to develop.
A. 对
B. 错
若电力系统有n个节点，若有m个PV节点，一般有多少个PQ节点？
A. n-m+1
B. n-m
C. n-m-1
D. m-1
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