The fish are dead because the polluted water.

Correction:

• C: "the polluted water" → "of the polluted water"
  Corrected sentence: The fish are dead because of the polluted water.

I have known Bob since five years.

Correction:

• B: "since" → "for"
  Corrected sentence: I have known Bob for five years.

The water and land around the chemical factory are serious polluted.

Correction:

• C: "serious" → "seriously"
  Corrected sentence: The water and land around the chemical factory are seriously polluted.