سؤال اليوم — اختبار STEP

Read the passage and answer the question. (1) In a country with no permanent rivers and very little rainfall, providing enough fresh water for a growing population is a constant challenge. For decades, Saudi Arabia has met much of this demand through desalination, the process of removing salt from seawater. The Kingdom is now one of the largest producers of desalinated water in the world, and coastal plants along the Red Sea and the Arabian Gulf supply cities hundreds of kilometres inland through long pipelines. (2) Early plants mostly used thermal desalination, in which seawater is heated until it evaporates, leaving the salt behind; the steam is then cooled to produce fresh water. This method is reliable but consumes enormous amounts of energy. Most new plants instead use reverse osmosis, which pushes seawater at high pressure through membranes with pores so small that salt cannot pass. Reverse osmosis typically needs much less energy than heating, which is why it has become the preferred choice. (3) Desalination still faces two main problems. The first is cost, since even efficient plants require large amounts of electricity. The second is brine, the very salty water left over after fresh water has been extracted. If brine is released carelessly, it can harm marine life near the outlet. (4) Engineers are working on both issues. Some new plants are powered partly by solar energy, which is abundant in the region, and researchers are exploring ways to extract useful minerals from brine instead of discharging it into the sea. If these efforts succeed, desalination could become both cheaper and cleaner in the years ahead. Question: According to paragraph 2, why has reverse osmosis become the preferred method?