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.
What is the main idea of the passage? （no more than 5 words）
_____________________________________________________________________________
When do floods occur? (no more than 6 words)
_____________________________________________________________________________
What causes catastrophic floods? (no more than 8 words)
_____________________________________________________________________________
Fill in the blank in Paragraph 3 wit proper words. (no more than 4 words )
_____________________________________________________________________________
What does the word “They” (Line 1, Paragraph 2) probably refer to? (1 word)
_____________________________________________________________________________