XDF 补充教材 section 25-Q6

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 begining to raise alarms.

B. at a cost raising water bills so high that

D. whose cost will raise water bills so high that

偶知D是正确答案，请问B错在哪里？谢谢!!!!!

gwd 7-15  
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