【问】 “Did Mary enjoy her trip on the ocean liner? ”“No，she stayed in her cabin while the shipwas____.”
 A. over the sea B. in the sea C. to sea D. at sea(TOEFL，71.1)
 为什么这里应当选D(at sea)?它与at the sea和in the sea，on the sea有何区别?
 【答】 at sea是个固定用法，意思是“在海上(航行)”或“航行中，航海旅行中”：
 1. When he woke up，the ship was at sea. (Palmer)
 当他醒来时，船正在航行。
 2. By the second day the ship was well out at sea. (DAI)
 到第二天船早已航行在海上了。
 3. They had first met at sea. (DAI)
 他们是航海旅行中第一次相遇的。
 at sea的比喻用法，作“茫然”(be puzzled)解：
 1. He was all at sea when he began his new job.(OSD)
 当他刚开始干他的新工作时，他一片茫然。
 2. I don’t understand politics： I’m (all) at sea when people talk about the government.(LDCE)
 我不懂政治;当大家谈到政府事务时，我茫然如坠五里雾中。
 at the sea的意思是“在海边”(at the seaside)。
 He spent the summer at the sea.
 他在海滨度过了夏天。
 on the sea是指“在岸边”：
 1. Brighton is on the sea. (OSD)
 布赖顿座落在海岸边。
 2. a town with a beautiful position on the sea(LDCE)
 位处海滨，地点甚佳的一座城镇
 in the sea是指“在海里”：
 The small boat struggled in a turbulent sea. (FWF)
 那只小船在波涛汹涌的海洋里拼搏航行。