GWD20-15，难题。

The city has proposed a number of water
treatment and conservation projects the cost of which raises water bills
high enough so that even environmentalists are beginning to raise alarms.

        
1. the cost of which raises water bills high enough so that

        
2. at a cost raising water bills so high that

        
3. at a cost which raises water bills high enough so

        
4. whose cost will raise water bills so high that

        
5. whose cost will raise water bills high enough so that

The city has proposed a number of water
treatment and conservation projects the cost of which raises water bills
high enough so that even environmentalists are beginning to raise alarms.

1. the cost of which raises water bills high enough so that
/ high enough so that is not an idiomatic expression
  
2. at a cost raising water bills so high that
/ raising can also describe the city
  
3. at a cost which raises water bills high enough so
/ high enough so is not an idiomatic expression
  
4. whose cost will raise water bills so high that

  
5. whose cost will raise water bills high enough so that

        high enough so that is not an idiomatic expression