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It is suggested in the passage that air pollutants ----.
When air pollution, including acid rain, is combined
with other environmental stresses, such as low winter
temperatures, prolonged droughts, insects, and
bacterial, fungal, and viral diseases, it can cause
plants to decline and die. More than half of the red
spruce trees in the mountains of the northern United
States have died since the mid-1970s. Other tree
species, such as sugar maples, for example, are also
dying. Many still-living trees are exhibiting symptoms
of forest decline, characterized by a gradual
deterioration and often eventual death. The general
symptoms of forest decline are reduced vigour and
growth, but some plants exhibit specific symptoms,
such as yellowing of needles in conifers. Air
pollutants may or may not be the primary stress that
results in forest decline, but the presence of air
pollution lowers plant resistance to other stress
factors. When one or more stresses weaken a tree,
then an additional stress may be enough to cause
death.