The environment is everything that surrounds us: plants, animals, buildings, country, air, water--literally everything that can affect us in any way. The environment of a town, with its buildings and traffic and its noise and smells, where everyone is on top of everyone else, is a far cry from that of the countryside, with its fields and crops, its wild and domestic animals and its feeling of spaciousness. And the environment differs in different parts of the world. Ecology is the science of how living creatures and plants exist together and depend on each other and on the local environment. Where an environment is undisturbed, the ecology of an area is in balance, but if a creature is exterminated or an unfriendly species introduced, then the ecology of the district will upset--in other words, the balance of nature will be disturbed. Man is a part of the environment and has done more to upset the ecology during his short span on earth than any other living creature. He has done this by his ignorance, his greed, his thoughtless foolishness and wastefulness. He had poisoned the atmosphere and polluted both land and water. He has squandered the earth’s natural resources with no thought for the future, and has thought out the most savage ways of killing his fellow men--and every other sort of life at the same time. Since man has done so much damage, it is up to man to try to put matters right--if it is not already too late. If there is to be any remedy for our ills, that remedy ultimately lies in the hands of the young, and the sooner they start doing something about it, the better. One of the main causes of the earth’s troubles is that the world is overpopulated and that this overpopulation is growing at an ever-increasing rate. At the same time we are using up our natural resources--fuels and mineral ores--at an ever increasing rate with no hope of replacing them. For many years the earth has been unable to provide enough food for these rapidly expanding populations and the position is steadily worsening since the fertility of some of our richest soils has been lost and vast areas that were once fertile lands have turned into barren deserts. And the trouble with deserts is that they tend to creep outwards on to the fertile soils. What is now the northern Sahara Desert was the cradle of the civilized world 2,500 years old. Even at this moment many of the earth’s natural treasures are being destroyed. Many valuable animals and plants are being killed off, and it is becoming increasingly difficult to grow enough food to preserve much of the earth’s population from starvation. The situation is getting out of hand. Time is running out. But with your help, we may be able to reserve the trends which threaten our very existence. Which of the following problems is NOT mentioned in the passage

A. Growing population.
B. Food shortage.
C. Human illness.
D. Extinction of some species.