Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks.

For all the talk of what humankind has done to cause destruction to the planet, we should not forget that nature itself is a (39) ______ force, capable of doing damage on a huge scale. The greenhouse effect and the hole in the ozone layer are indeed made worse by the industrialized society we live in. (40) _______, we must remember that there are certain species of plants and animals which, quite naturally, (41) ______ off gases that are very harmful to the atmosphere. And only 50 years ago in London, hundreds of people died from the smog (42) ______ hung over the River Thames. What is interesting here is that smoke and gases from industry, vehicles and fossil fuels were ingredients in this deadly mixture. The (43) ______ vital ingredient, though, was the completely natural fog, and who would have thought that could be harmful? In the early days of the industrial revolution, poets, painters and philosophers described how the polluted urban skies made them think of the end of the world