____________________
is
the
introduction
of
____________________
materials
into
the
____________________
.
These
harmful
____________________
are
called
____________________
.
Pollutants
can
be
natural
,
such
as
____________________
ash
.
They
can
also
be
created
by
____________________
activity
,
such
as
____________________
or
____________________
produced
by
____________________
.
Pollutants
____________________
the
quality
of
air
,
water
,
and
land
.
