Câu hỏi:

Read the passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 20 to 24.

Tsunami is a Japanese word that means harbor wave and is used as the scientific term for seismic sea wave generated by an undersea earthquake or possibly an undersea landslide or volcanic eruption. When the ocean floor is tilted or offset during an earthquake, a set of waves is created similar to the concentric waves generated by an object dropped into the water. Most tsunamis originate along the Ring of Fire, a zone of volcanoes and seismic activity, 32,500 km long that encircles the Pacific Ocean. Since 1819, about 40 tsunamis have struck the Hawaiian Islands. tsunami can have wavelengths, or widths, of 100 to 200 km, and may travel hundreds of kilometers across the deep ocean, reaching speeds of about 725 to 800 kilometres an hour. Upon entering shallow coastal waters, the wave, which have been only about half a metre high out at sea, suddenly grows rapidly. When the wave reaches the shore, it may be 15 m high or more. Tsunamis have tremendous energy because of the great volume of water affected. They are capable of obliterating coastal settlements. Tsunamis should not be confused with storm surges, which are domes of water that rise underearth hurricanes or cyclones and cause extensive coastal flooding when the storms reach land. Storm surges are particularly devastating if they occur at high tide. A cyclone and accompanying storm surges killed an estimated 500,000 people in Bangladesh in 1970. The tsunami which struck south and southeast Asia in late 2004 killed over 200 thousand people. 

What does the word “concentric” in paragraph 1 mean?