Halong bay is situated in Quang Ninh province, in the northeast region of Vietnam. It is one of (26) ________ most extraordinary natural wonders and is the perfect destination for any tourist. It is said that Ha Long Bay is a magical place, attracting more tourists than ever since.

Viewed from any angle, Ha Long Bay looks like a work of art. It has 1,969 islands concentrated in two main zones: the southeast (27) _________ the southwest. At the centre of the islands, there are (28) _________ caverns, such as Thien Cung, Dau Go, Sung Sot, and Tam Cung. For those interested in history, a visit to Van Don Island, Poem Mountain, and the Bach Dang River is a must. It is now known that Ha Long was one of the places (29) _________ humans first existed. With its long history, astounding natural beauty and the open (30) _________ of the people here, the experience of visiting Halong is unforgettable.

The word "they" in paragraph 3 refers to __________.