SC Whose Usage

[junkstuff710]

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.

A. the cost of which raises water bills high enough so that
B. at a cost raising water bills so high that
C. at a cost which raises water bills high enough so
D. whose cost will raise water bills so high that
E. whose cost will raise water bills high enough so that

Can anyone explain why there needs to be a whose in the answer choice. What is wrong with the "cost of which"?