Something must be done to prevent the factory ________ poisonous gases so as to keep the city ________．

1. A.
   sending out; from polluting
2. B.
   send out; polluting
3. C.
   from sending out; from polluting
4. D.
   sending out; from being polluted

 Something must be done to prevent the factory ________ poisonous gases so as to keep the city __________.

   A. sending out; from polluting         B. send out; polluting

   C. from sending out; from polluting     D. sending out; from being polluted