【题目】There’s no snake known that will habitually attack human beings unless _______ with its life.
A. threaten B. threatened
C. threatening D. having threatened
在线课程【答案】B
【解析】
考查过去分词。句意：没有一条蛇会习惯性地攻击人类，除非它的生命受到威胁。此处句子主语snake和threaten之间是被动关系，所以unless和过去分词threatened构成条件状语，故选B。