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
我选了B，选的时候就在B和D之间纠结，不知道B错在哪里，请NN们指点迷津~~
谢谢！ | 
