【正确答案】 C

【答案解析】[解析] One thing that really concerns water resource analysts is how much water agriculture uses--much more than all other water-using sectors of society. One of our greatest concerns is the very high use of water by irrigation. Of course, some forms of irrigation use water more efficiently than others do. The efficiency of water use varies by region, crop, agricultural practice, and irrigation technology. The least efficient types of irrigation are the traditional surface methods, such as field flooding. It takes a lot of water to flood a field. The water collects into ponds or basins, but then most of it either evaporates into the air or passes down through the soil into groundwater. This means that less than half of all the water applied to the field is actually used by the crop. The rest is lost to evaporation or to groundwater. What is the lecture mainly about? The professor mainly discusses the high use of water by irrigation. He says: One of our greatest concerns is the very high use of water by irrigation; The least efficient types of irrigation...field flooding. It takes a lot of water to flood afield.