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. Although

C. If

D. Once

答案：

A. As

(答案→)B. Although

C. If

D. Once

解析：句意：尽管地球的表面覆盖着水，但是淡水还是稀少的和宝贵的。as因为；although尽管； if 如果； once一旦。根据句意故选B。