The text is for questions 17 - 19.

    A natural disaster is a terrible accident, e.g. a great flood, a big fire or an earthquake. It usually causes great suffering and loss of a large sum of money. The casualties are injured or died. Some people are homeless and need medical care.

    Floods occur when the water of rivers, lakes, or streams overflow their banks and pour onto the surrounding land. Floods are caused by many different things. Often heavy rainstorms that last for a brief can cause a flood. But not all heavy storms are followed by flooding. If the surrounding land is flat and can absorb the water, no flooding will occur. If, however, the land is hard and rocky, heavy rain cannot be absorbed. Where the banks are low, a river may overflow and flood adjacent lowland.

    In many part of the world flood are caused by tropical storms called hurricanes or typhoons. They bring destructive winds of high speed, torrents of rain, and flooding. When a flood occurs, the destruction to surrounding land can be severe. Whole villages and towns are sometimes swept away by water pouring swiftly over the land. Railroad track blocked and uprooted from their beds. Highways are washed away.

    When a building caught fire, the firemen pitched in to help battle the blaze. Before the pumps were invented, people formed bucket brigades to fight fires. Standing side by side, they formed a human chain from the fire to nearby well or river. They passed buckets of water from to hand to be poured on the flames.

    The damage of the fire did depend a great deal on where it happened. In the country or a small village, only a single house might burn down. But in crowded cities, fire often destroyed whole blocks and neighborhoods before being controlled. (Soal UN SMA/MA IPA 2011/2012)

17. What can possibly prevent rivers and lakes from overflowing?