What we don't know or refuse to recognize is that modern man has been altering his total environment so swiftly and suddenly that the whole "great chain of life" on this planet is in danger.
All of us live on a tiny spaceship carrying with it its own limited resources for survival.What we have now is all we will ever have to keep us alive.
But man has been harming this system. He cannot help it. Everything we do alters our environment: the ways we grow food and build shelter and create what we call "culture" and "civilization".
World population is growing at such a rate that by the year 2050 nearly seven billion people will live on the earth.Industrialization has added its own burden to the population pressure. The more we produce and consume, the more waste products we discharge into the air and water and land around us, where they do not disappear, but last forever in one form. or another. Great progress is being made in the productivity of our soil, yet agriculture is now taking more nutrients (营养) from it than are being replaced each year.
"Ecology", the study of the relationship between life systems and their environment, is what everybody on this planet must now start thinking about quickly if we are to avoid destructive changes within the closed system of our spaceship.
It seems to the writer that ______.

A. it is easy to do harm to our environment
B. the planet on which man lives is too small for man to survive
C. man should not create his culture and civilization too rapidly
D. man does not want to admit the danger of what he has done to the environment