16.I often hear some old people say that when they were young,the rivers and lakes were very clean.They swam in them and even drank the water directly from them.There(1)
be lots of fish and other living things in the rivers. But now(2)
the pollution,there aren't many fish left.In some of the rivers there are no fish at all.The water has been polluted and no longer(3)
for swimming or drinking as a lot of(4)
are present in the water.In some places it is not safe to eat the fish. Water pollution is mainly caused by(5)
from factories and cities.They throw all the waste into the river(6)
cleaning them. Gases from factories and cars go up into the(7)
,and the wind carries them far away.Later they come down with the rain.That is acid rain (酸雨). Fish eggs and young fish are(8)
by even a little acid rain in the water.Acid rain is becoming a big(9)
all over the world today.Canada and some countries in northern Europe have the problem.In Canada,for example,a lot of trees are (10)