阅读下面的短文并回答问题，然后将答案写到题后的横线上（请注意问题后的词数要求）。
1 A flood is the overflow of a huge amount of water onto normally dry land. Flooding occurs when the overflowing water submerges (淹没) land, and it causes a lot of problems. It is a cruel and violent expression of water.
2 Floods are often deadly and damaging. They kill lots of people, damage houses and crops, and cause extensive destruction. In broader terms, floods are of two types: natural floods and catastrophic floods. Natural floods are the floods that are caused naturally by the overflow of a huge volume of water, from rivers, lakes, oceans, or by heavy rain or downpours, hurricanes, and so on. Catastrophic floods are floods that are caused by some significant (重大的) or unexpected events, for instance, dam breakages.
3 Heavy rainfall is one of the major causes of floods. The level of water in rivers or lakes rises due to heavy rainfall. When the level of water rises above the river banks or dam, the water starts overflowing, which causes floods. Floods occur more in the regions that .
4 Floods are also caused due to heavy snow melting. Global temperatures are rising due to global warming. The rising temperatures are making the snow caps melt faster. Continuous and fast melting snow raises the level of water in the oceans, which consequently raises the level of water in rivers, and when the level of water in rivers rises above the river banks, it causes floods.
5 Generally, floods occur more in low-lying areas or areas below sea level. One of the main reasons is that rivers flow slowly in these areas. The volume of water increases in low-lying areas. When the level of water rises in these regions, it causes floods.
6 Sometimes floods are caused due to poor dams that cannot hold great volume of water. Therefore, there are always different causes of floods. However, floods caused by humans can be avoided. Humans should let the nature go its own way.
【小题1】What is the main idea of the passage? （no more than 5 words）
【小题2】When do floods occur? (no more than 6 words)
【小题3】What causes catastrophic floods? (no more than 8 words)
【小题4】Fill in the blank in Paragraph 3 wit proper words. (no more than 4 words )
【小题5】What does the word “They” (Line 1, Paragraph 2) probably refer to? (1 word)
What causes floods.
网友回答
(答案→)What causes floods.
解析：洪水是大量的水流向平常干燥的地区。当大量的水淹没陆地时，就产生了洪水灾害。洪水通常是致命的、具有破坏性的。可以分为自然性和灾害性洪水。造成洪水的原因有很多。
【小题1】文章第一段介绍了洪水的概念；第二段讲的是洪水的分类，接下来几段讲的是洪水产生的原因。Heavy rainfall；heavy snow melting；low-lying areas or areas below sea level；poor dams等都是洪水产生的原因，文章主要讲的是造成洪水灾害的原因，故填What causes floods.。
【小题2】根据第一段“Flooding occurs when the overflowing water submerges (淹没) land”可知，当过多的水淹没地面时，就会产生洪水灾害。故填When the overflowing water submerges land.。
【小题3】根据第二段“Catastrophic floods are floods that are caused by some significant (重大的) or unexpected events, for instance, dam breakages.”可知，灾害性洪水是由一些重大或突发事件引起的洪水。故填Some significant or unexpected events, like dam breakages.。
【小题4】根据第三段“Heavy rainfall is one of the major causes of floods.”可知，本段讲的是大雨，所填的与大雨有关，故填get heavy rainfall。
【小题5】根据第二段“Floods are often deadly and damaging.”可知，代词they指的是Floods。故填Floods.。