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 from 17 to 21.

            Nowadays people are more aware that wildlife all over the world is in danger. Many species of animals are threatened, and could easily become (17) ______if we do not make an effort to protect them. There are many reasons for this. In some cases, animals are hunted for their fur or for other valuable parts of their bodies.

            Some birds, (18) ______as parrots are caught alive and sold as pets. For many animals and birds, the problem is that their habitat - the place where they live - is disappearing. More (19) ______is used for farms, for houses or industry, and there are fewer open spaces than there once were. Farmers use powerful chemicals to help them (20) ______ better crops, but these chemicals pollute the environment and harm wildlife. The most successful animal on earth - human beings - will soon be the only ones left, (21) ______ we can solve this problem.