most of the earth’s surface is covered by water fresh water is very rare and precious.A

问题补充：

most of the earth’s surface is covered by water, fresh water is very rare and precious.

A. As

B. Once

C. If

D. Although

答案：

A. As

B. Once

C. If

(答案→)D. Although

解析：本道题考查让步状语从句。一定要看懂：虽然地球表面覆盖着水，但是纯净水仍然很稀有和珍贵。选择D.