参考答案

正确答案：D
解析：关键语句：“The researchers say major irrigation systems were built in the 1980s．The systems took water from two rivers that flow into Lake Chad．The Chari and Logone rivers carry most of the water that enters the lake．The study showed the increased irrigation reduced the flow in the two rivers．Climate changes also were responsible for the reduction．Today the flow of the two rivers has been reduced by almost seventy—five percent．”可见一方面是湖区水源抽取过多，另一方面是天气情况变得恶劣，所以D正确。