___ 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

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

___ 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

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

1. A.
   As
2. B.
   Once
3. C.
   If
4. D.
   Although