参考答案

正确答案：D
解析：根据题干关键词cause，disaster定位到原文第一段最后一句：This disaster is threatening the nation's food security. 以及第二段的前半部分：Politicians blame the drought. But it's much more than that. Mismanagement and gross policy neglect of irrigation have helped create the water shortages. 可知，由客观和主观原因导致的缺水问题是澳大利亚乳制品业下滑的主要原因，D)项符合原文。