The unknown world's mystry The sea is the biggest unknown part of our world. It covers seventy-one percent of the earth. There is still much to be discovered about this vast blanket of water. If the waters of the ocean could be removed, the sea floor with its wide valleys, irregular mountains and rivers in the sea would be an unbelievable sight. Tourism under the deep blue sea Tourists can now explore the world beneath the waves without ever getting wet. From tourist submarines   to underwater dining, undersea tourism is making a big splash. Tourist submarines The popularity of submarine tours has grown rapidly in the last decade. Now, tourist submarines operate in more than 20 locations worldwide, including the waters around Okinawa and Taiwan's Green Island. The submarines carry nearly 2 million tourists each year. Large windows offer passengers spectacular views as the submarines explore colorful coral reefs and sunken ships. Most one-hour tours cost about US $80. Under water dinning For a more upscale experience, tourists can enjoy fine dining under the sea. Last summer the Hilton Maldives Resort opened the world's first underwater restaurant, Ithaa. Diners walk down a staircase into the restaurant, which rests 35 feet beneath the waves of the Indian Ocean. Transparent walls surround the dining area, and you can treat guests to a panoramic   ocean view. But this excellent view doesn't come cheap. The tasting menu costs US $200 per person. Be sure to make reservations early, too. The restaurant seats only 14 people. The walls of the restaurant Ithaa are most probably made of_.
stone brick glass steel
Question: The unknown world's mystry The sea is the biggest unknown part of our world. It covers seventy-one percent of the earth. There is still much to be discovered about this vast blanket of water. If the waters of the ocean could be removed, the sea floor with its wide valleys, irregular mountains and rivers in the sea would be an unbelievable sight. Tourism under the deep blue sea Tourists can now explore the world beneath the waves without ever getting wet. From tourist submarines   to underwater dining, undersea tourism is making a big splash. Tourist submarines The popularity of submarine tours has grown rapidly in the last decade. Now, tourist submarines operate in more than 20 locations worldwide, including the waters around Okinawa and Taiwan's Green Island. The submarines carry nearly 2 million tourists each year. Large windows offer passengers spectacular views as the submarines explore colorful coral reefs and sunken ships. Most one-hour tours cost about US $80. Under water dinning For a more upscale experience, tourists can enjoy fine dining under the sea. Last summer the Hilton Maldives Resort opened the world's first underwater restaurant, Ithaa. Diners walk down a staircase into the restaurant, which rests 35 feet beneath the waves of the Indian Ocean. Transparent walls surround the dining area, and you can treat guests to a panoramic   ocean view. But this excellent view doesn't come cheap. The tasting menu costs US $200 per person. Be sure to make reservations early, too. The restaurant seats only 14 people. The walls of the restaurant Ithaa are most probably made of_.
A. stone
B. brick
C. glass
D. steel
Answer: