_______(17)
A.looking
B.thinking
C.considering
D.wondering

_______(18)
A.balance
B.review
C.survey
D.assess

_______(20)
A.regarded
B.valued
C.recognized
D.understood

_______(22)
A.about
B.in
C.into
D.by

The visitors from the east coast mentioned in the passage thought that Los Angeles ____________ .
A.was not accurately portrayed by Hollywood images.
B.lacked good suburban areas in which to live.
C.had an excessively large population.
D.was not really a single city.

It can be inferred from the first paragraph that ____________ .
A.water vapor is an air pollutant in localized areas
B.most air pollutants today can be seen or smelled
C.the definition of air pollution will continue to change
D.a substance becomes an air pollutant only in cities

The word "altered" in line 11 is closest in meaning to ____________ .
A.eliminated
B.caused
C.slowed
D.changed

According to the passage, which of the following is true about human-generated air pollution in localized regions?
A.It can be dwarfed by nature's output of pollutants in the localized region.
B.It can overwhelm the natural system that removes pollutants.
C.It will damage areas outside of the localized regions.
D.It will react harmfully with naturally occurring pollutants.

According to the passage, the numerical valued of the concentration level of a substance is only useful if ___________ .
A.the other substances in the area are known
B.it is in a localized area
C.the naturally occurring level is also known
D.it can be calculated quickly

Which of the following is best supported by the passage?
A.To effectively control pollution local government should regularly review their air pollution laws.
B.One of the most important steps in preserving natural lands is to better enforce air pollution laws.
C.Human activities have been effective in reducing air pollution.
D.Scientists should be consulted in order to establish uniform limits for all air pollutants.