求助！一道语法题！非常困惑！！！

littleblues

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.

B.  at a cost raising water bills so hight that
D.  whose cost will raise water bills so high that

为什么B 不能选？ 而选D ？    谢谢！！

littleblues

哦！！！ 有道理！！ 明白！ 谢谢！

fuhongler

介词短语做状语的可能性比较大，在这里的at a cost有修饰动词proposed的嫌疑，

不如d明确

greedier

B基本不顺吧。at a cost和raising water bills之间没有恰当连接词.可以是at a cost which raises/will raise water bills...