Don’t use words ,expressions or phrases only to people with
specific knowledge.
A. being known
B. to be known
C. known
D. having been known
网友回答
A. being known
B. to be known
(答案→)C. known
D. having been known
解析：这题考查过去分词做定语的用法：words ,expressions or phrases是被大家知道，所以用过去分词，句意是：不要用些只是有专业知识的人才知道的单词短语和表达法。选C。