Read the passage and choose the best answer.

The tourist industry is considered to be (23)____ largest industry. Before 1950, one million people traveled abroad each year but by 1990s, the figure had risen to 400 million every year. Such large (24) ____of tourists; however, are beginning to cause problems. For example, on the Alps the skiers are destroying the mountains they came to enjoy. Even parts of Mount Everest in the Himalayas are reported to be covered (25) ____ old tins, tents and foods that have been thrown away.  But at a time (26)____ we have greater freedom to travel than ever before, more and more people are asking how they can enjoy their holidays without (27)____ problems by spoiling the countryside. Now there is a new holiday guide called “Holidays that don’t cost the earth”. It tells you how you can help the tourist industry by asking your travel agent the right before you go on holiday.