Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the following questions from 39 to 43.

Can you imagine waking up each day in a house that is gently rocked by the movement of water, having a water park as a garden and getting to work by water taxi? Well, for some people this vision of the future may become a reality. It is a commonly accepted fact that as the Earth's temperature rises, the ice caps at the North and South Poles will melt, causing the sea level to rise.

So, where does this leave the many countries in the world which have large sections of their populations living in areas which lie at or below sea level? The Netherlands, for example, is one of the most highly populated, low-lying countries in the world and has been battling with the problem of flooding for years. Further rises in the sea level would mean much of the country will disappear under water.

Inventive architects may have come up with a solution to the problem: a floating house actually built on the water. The basic idea is that houses will have a base filled with air and a way of anchoring them to the ground. This will allow the houses to float upwards whenever the sea level rises and sink back down when the sea level drops. So, are floating houses the homes of the future? For some people there may be no alternative; for others it will be through choice. After all, who says you have to live on dry land anyway?

The word their in the second paragraph refers to_______.