Hurricanes are the most violent storms on Earth. People call these storms by other names, such as typhoons or cyclones, (17) ______ on where they occur. The scientific term for all these storms is tropical cyclone. Only tropical cyclones that form over the Atlantic Ocean or eastern Pacific Ocean are (18) ______ "hurricanes." Tropical cyclones are like giant engines that use warm, moist air as fuel. That is why they form only over warm (19) ______ waters near the equator. The warm, moist air over the ocean rises upward from near the surface. Because this air moves up and away from the surface, there is less air left near the surface. Another way to say the same thing is that the warm air rises, causing an area of lower air (20) ______ below. Air from surrounding areas with higher air pressure pushes into the low-pressure area. Then that "new" air becomes warm and moist and rises, too. As the warm air continues to rise, the surrounding air swirls in to (21) ______ its place. As the warmed, moist air rises and cools off, the water in the air forms clouds. The (22) ______ system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the surface. 17. A. discounting B. depending C. challenging D. separating 18. A. called B. paused C. dated D. signed 19. A. canal B. spring C. river D. ocean 20. A. volume B. amount C. pressure D. pollution 21. A. land B. take C. bend D. make 22. A. dull B. frank C. cruel D. whole