GWD Q 15 help

Why D is the answer, not B?

Isn't is right that projects should be followed by "which cost" not "whose cost"?

Q15:

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

   Answer: