Điền từ vào ô trống kèm giải thích The tourist industry is considered to be the worlds largest industry . Before 1950 about one million people traveled abroad each year but by the 1990s the figure had risen to over 400 million a year Such large (1) of tourist ,however,are beginning to (2) problems.for example,in the Alps the skiers are destroying the mountains they came to enjoy.Even parts of Mont Everest in the Himalayas are reported to be covered (3) old tins ,tents,and food that have been thrown away But at a time when we have greater freedom to travel than ever before ,more and more are asking how they can enjoy their holiday without causing problems by spoiling the countryside Now there is a new holiday guide called Holidays That Dont Cost The Earth .It tells you how you can help the tourist industry by asking your travel agent (4) you go on holiday. (1) A. number B. many C. numbers D. a lot (2) A. lead B. result C. cause D. have (3) A. on B. with C. in D. by (4) A. before B. after C. while D. when