单选题 As indicated in the passage, the water problem ______.

A、 has been exaggerated by some experts in the field
B、 is underestimated by government organizations at different levels
C、 poses a challenge to the technology of building reservoirs
D、 is already serious in certain parts of the world.

【正确答案】 D

【答案解析】依据为文章第一行起相关内容。