三、阅读短文内容，在空白处填入一个适当的单词或括号内单词的正确形式。
Son Doong Cave(韩松洞) in Vietnam is one of the largest caves in the world. It is famous as one of the 20 record-breaking 21. (nature) wonders.
Son Doong Cave is 22. (locate) in the heart of a national park in Vietnam. It remained undiscovered until a local 23. (farm) named Ho Khanh found the small opening to the cave in 1991. He told the story of the cave to other villagers, 24. no one thought it was true.
Years later, a group of British 25. (explorer) heard about Khanh's story. They learnt 26. Khanh that there was a strong, cool wind and the sound of water coming out of the opening. These were signs showing that there was a large cave. They asked Khanh to help 27. (they) find the cave. But Khanh forgot where the opening was. In 2009, Khanh and some people finally found the cave 28. (successful) again. They went inside and were very surprised. They found a large underground river 29. (run) through the cave! They also found a forest. It is 200 metres below the ground. When the sun shines into the cave, it is really beautiful.
If you want to visit the cave, you need 30. (cross) rivers, climb up walls and get very muddy(沾满泥的)! How difficult it is!

21.natural
22.located
23.farmer
24.but
25.explorers
26.from
27.them
28.successfully
29.running
30.to cross