Câu hỏi:

Read the passage carefully and choose the correct answer.

SON DOONG CAVE

Located in the limestone mountains of Phong Nha - Ke Bang National Park, Son Doong Caves was first found in 1991 by Ho Khanh, a local man. In 2009, the cave was explored and published by a group of scientists from British Cave Research Association. Son Doong Cave was formed from 2 to 5 million years ago, when the river flowed through the limestone mountain area and was buried. The water eroded limestone and created an underground tunnel. In soft limestone areas, the collapse of tunnel ceiling formed large holes which became giant domes afterward. The cave is about 200m high, 200m wide and at least 8.5 km long. Especially, Son Doong consists of two entrances, which is unique among explored caves in Phong Nha ~ Ke Bang National Park. Exploring the cave,  visitors were surprised with spectacular scenery of numerous stalactites of which some giant stalagmites are more than 70 metres high. Deep inside the cave exists a grandiose tropical jungle called “Garden of Eden” by the explorers. The jungle is home to diversified system of fauna and flora. Besides, the cave possesses a 2.5-kilometre underground river. Not far from the “Garden of Eden” lies an enormous “pearl collection” consisting of tens of thousands of small stone pieces in dry ponds, contributing to magnificent beauty of the cave.

The reason why Son Doong Cave was formed is that ________.