求助GWD7-15,拜托了

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
疑惑：1：
         正确选项是D选项
         答案解释说B选项，at a cost of doing即使这个选项把这一点错误改掉后，仍然不如E选项好，因为E的定语从句很明
   确地指出了修饰对象
   请问，whose的修饰对象是什么，这里不是没有人么？
2.at a cost of doing 是正确用法吗？
谢谢啦，希望各位大神帮帮忙！