阅读下列短文，从每题所给的A、B、C、D四个选项中选出最佳选项。
A
The Best Caves in The World
Hang Son Doong, Vietnam
Natural caves don't come much larger than Hang Son Doong, close to the border between Laos and Vietnam. This cave possesses the largest cross-section of any known cave on the planet, a vast area that is difficult to describe. Supposedly, a Boeing 747 could fly through without damaging its wings, but that doesn't really do justice to the vastness of Hang Son Doong. The stalactites （钟乳石） here are pretty massive too, with some reaching up to
80 metres.
Waitomo Caves, New Zealand
Glowworms （萤火虫） are there, as far as the eye can see. Okay, not literally, but the Waitomo Cave system on New Zealand's North Island is best-known for the fluorescent fauna that light up the walls, giving it the not-particularly-creative but completely acceptable “Glowworm Caves” nickname. They are more accessible than other caves on this list, with rafting and adventure tours available to those looking for something a little more
thrilling.
Mammoth Cave, the USA
If you have certain expectations from somewhere called “Mammoth Cave”, that is entirely understandable. Mammoth Cave in Kentucky is the world's longest known cave system, an incredible 420 miles of underground wonder. That’s twice as long as the next longest, by the way, although it isn't unusual for the USA to go all out on
such things.
Reed Flute （芦笛） Cave, China
Named after the reeds that grow outside, which are used to make flutes, obviously, the Reed Flute Cave's walls are covered with inscriptions from centuries gone by—if evidence was needed that people have been paying attention to this place for a long old time. The inside part of the cave is also lit up by multicoloured lights, giving it
a real otherworldly theme that adds weight to the nickname.
21. What is special about Hang Son Doong?
A. It was once a base of a factory. B. It is the deepest cave in the world.
C. It has the highest stalactites in the world. D. It owns the largest cross-section in the world.
22. Which of the following can be much easier to enter?
A. Hang Son Doong. B. Waitomo Caves.
C. Mammoth Cave. D. Reed Flute Cave.
23. Which country probably has the longest cave system in the world?
A. Vietnam. B. New Zealand. C. The USA. D. China.
