英语语法,请教一下解析的过程

英语语法,请教一下解析的过程 ( ) to find words to express their gratitude,they silently grasped their hands.
选项:
a、they are unable
b、been unable
c、unable
d、as unable
答：C
译：不知道如何表达他们的感激之情,他们静静地紧握双手(抱拳表示感谢,一定是中国人.）
析：形容词短语作状语,表示原因=As they were unable to .
评：D项也是可以的.
我们知道,分词短语（包括形容词短语）可以作状语,实质是省略了连词+主+be动词.有时候为了明确是什么状语,前面保留连词如：When heated,water will become vapour.= When it is heated,...
If it is necessary,I will help you.==》I will help you if necessary.
As unable to ...=As they were unable to ...,
本题设计有误.