Câu hỏi:

Mark the letter A, B, C or D on your answer sheer to indicate the correct answer to each of the following questions.

Son Doong Cave has become more famous after the American Broadcasting Company (ABC) aired a live programme featuring its magnificence on "Good Morning America' in May 2015. Located in Quang Binh Province, Son Doong Cave was discovered by a local man named Ho Khanh in 1991, and became known internationally in 2009 thanks to British cavers, led by Howard Limbert. The cave was formed about 2 to 5 million years ago by river water eroding away the limestone underneath the mountain. It contains some of the tallest known stalagmites in the world - up to 70 metres tall. The cave is more than 200 metres wide, 150 metres high, and nearly 9 kilometres long, with a cavern is big enough to fit an entire street inside them. Son Doong Cave is recognised as the largest cave in the world by BCRA (British Cave Research Association) and selected as one of the most beautiful on earth by the BBC (British Broadcasting Corporation). In August 2013, the first tourist group explored the cave on a guided tour. Permits are now required to access the cave and are made available on a limited basis. Only 500 permits were issued for the 2015 season, which runs from February to August. After August, heavy rains cause river levels to rise and make the cave largely inaccessible.  

What happened to Son Doong Cave in May 2015?