Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 19 to 24.

Ha Long Bay is (19) ______ UNESCO World Heritage Site and popular travel (20) ______ in Quang Ninh Province, Vietnam. The bay features thousands of limestone karsts and isles in various shapes and sizes. Ha Long Bay is a centre of a larger zone which (21) ______ Bai Tu Long Bay to the northeast, and Cat Ba Island to the southwest.

Ha Long Bay has an area of around 1,553km2, including 1,960 - 2,000 islets, most of (22) ______ are limestone. The core of the bay has an area of 334km2 with a high density of 775 islets. The limestone in this bay has gone through 500 million years of formation in different conditions and environments. The (23) ______ of the karst in this bay has taken 20 million years under the impact of the tropical wet (24) ______.

The geodiversity of the environment in the area has created biodiversity, including a tropical evergreen biosystem, oceanic and seashore biosystem. Ha Long Bay is home to 14 endemic floral species and 60 endemic faunal species.