I have never visited the Great Wall.—_____.

Why do you drink so much coffee? — Well, ________ it doesn’t keep me awake at night,I see no harm in it.[ ]

The cigarette end thrown away by a careless farmer ______ the big forest and brought us great loss.

The map is one of the best tools a man has __ he goesto a new place．

to nuclear radiation, even for a short time, may produce variants(变异)of genes in human bodies.[ ]