____ human beings live in large numbers, the waters are very likely to be polluted.

1. A.
   That
2. B.
   Where
3. C.
   Once
4. D.
   As

___ human beings live in large numbers, the waters are very likely to be polluted.

A．That　　　　　　 B．Where　　　　　　C．Once　　　　 D．As

_______human beings live in large numbers, the waters are very likely to be polluted.

A. That　　　 B. Where　　　　 C. Once　　　　　 D. As