阅读下列短文，从每题所给的A、B、C和D四个选项中，选出最佳选项。
A
Blue Grotto, Italy
Despite its name, it is actually not a grotto(洞穴)but a sea cave, about 60m long. What makes it one of the most beautiful caves in the world is the blue color, as its name suggests. When you enter inside, you can see it is filled with weird blue light. This color is formed by the unique combination of the rocks, the entrance, the clear water and the whole shape. In fact, you can’t identify where the source of the blue light is.
Son Doong Cave, Vietnam
This is known as the largest cave in the world, not because of its length—in fact, it is just about 5km long. But its size comes from its enormous passages and caves, reaching 200m in height. In addition, there is an underground river, making the cave even more impressive.
Marble Caves, Chile
Located far from any road, the caves are accessible only by boat. This is the most beautiful cave structure in the world, in which crystal clear water is literally lightened by dazzling(使目眩)blue ice walls. A magical, fabulous, unreal and almost unearthly sight! The caves are best seen from September to February and earlier in the morning when the sunlight streams inside.
Reed Flute Cave, China
Its name comes from the reeds beside its entrance that have been used for making flutes(长笛). It is not too long and not too deep-only about 500m in length. What makes this cave more special is the history. It has been known since ancient times. And when you enter inside, you can see a lot of inscriptions(碑文)dating back to medieval times, starting from the times of the Tang Dynasty (around 792 AD).
21．What is Blue Grotto most famous for?
A．Its long history. B．Its strange shape. C．Its clear water. D．Its amazing color.
22．When are the visitors advised to visit Marble Caves?
A．At noon in March. B．At night in October.
C．At sunrise in December. D．In the morning in August.
23．Which is the largest cave of the four?
A．The Blue Grotto. B．Son Doong Cave.
C．Marble Caves. D．Reed Flute Cave.
