WHAT DOES THE WORD “IT” IN THE LAST SENTENCE REFER TO

5. What does the word “It” in the last sentence refer to ?a. the polluted air b. the burning material c. the health problem d. the plant and animalChoose the item(A, B, C, D) that best answers the question about the passage:Books which give instructions on how to do things are very popular in the United States today. Thousands of these How-to books are useful: in fact, there are about four thousand books with titles that begin with the words “How-to’. One book, may tell you how to earn more money, another may tell you how to save or spend it, and another may explain how to give your money away.Many How-to books give advice on careers. They tell you how to choose a careerand how to succeed in it. If you fail, however, you can buy the book”How to Turn Failureinto Success”. If you would like to become very rich, you can buy the “How to Make a Millionaire”. If you never make any money at all, you may need a book called” How to Live on Nothing”.One of the most popular types of books is one that helps you with personal problem. If you want to have a better love of life, you can read “How to Succeed in Loveevery Minut of Your Life”. If you tred of books in happiness, you may prefer books whichgive step by step instruction on how to redecorate or enlarge a house.Why have How-to books become so popular/ probably because life has become so complex. Today people have far more free time to use, more choices to make, and more problem to solve. How-to books help people deal with modern life.