单项选择题 A tropical storm is building strength some distance from your ship. Waves are coming from the east，with periods increasing from 5 seconds to 15 seconds. The swell is from the east. Where was the storm when these new swells were generated? （）

单项选择题 A tropical cyclone has recurved and entered temperate latitudes. In the Northern Hemisphere when a large high pressure system lies north of the storm，what situation may occur?（）

单项选择题 A sylphon cell is a part of a（）.