【正确答案】 C

【答案解析】过去分词短语作时间状语，相当于when they were faced。本句的意思是：“在面对顽敌时，他们总是退却。”face表示“面临，正视”时有两种用法：be faced with面临，如They are all faced with the same problem．(他们都面临同样的问题)；用作及物动词，He faced the danger bravely．(他勇敢地面对困难。)