In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.
Q: Article: Waves are beautiful to look at,but they can destroy ships at sea,as well as houses and buildings near the shore.What causes waves? Most waves are caused by winds blowing over the surface of the water.The sun heats the earth,causing the air to rise and the winds to blow.The winds blow across the sea,pushing little waves into bigger and bigger ones. The size of a wave depends on how strong the wind is,how long it blows,and how large the body of water is.In a small bay big waves will never build up,but at sea the wind can build up giant,powerful waves. A rule says that the height of a wave (in meters)will usually be no more than one-tenth of the wind's speed(in kilometers).In other words,when the wind is blowing at 120 kilometers per hour,most waves will be about twelve meters of course,and some waves may combine to form giant waves that are much higher.In 1933 the United States Navy reported the largest measured wave in history.It rose in the Pacific Ocean to a height of thirty-four meters.
Answer: at sea
A:
The waves can build up _ .