A．Bacteria are as big as a pinhead.
B．Bacteria can not be seen by an ordinary microscope.
C．Bacteria are hard to see because they often live together.
D．Bacteria are very small so they can only be measured in microns.

A．Pollution causes a drop in the world’s average air temperature.
B．Solar radiation will causes a drop in the air temperature.
C．The ash of volcanic eruption will last for a long time.
D．The ash in the stratosphere has no direct influence on people on land.