请教小四英文题，谢谢

haizimama · 发表于 10-2-2011 15:45:56|来自：新加坡

在2010小四英文卷子有一题：
Crime has been increasing at an  (                   ) high rate in this neighbourhood.

1.alarm 2.alarmed 3.alarming  4.alarmingly

答案是4，alarmingly  小孩不理解这里为什么不用3.alarming?

在句酷上找到一句例句：This is now becoming an alarmingly common occurrence in the US.
不理解这里为什么用alarmingly 这个副词，请各位帮忙解答。谢谢