____ has already been pointed out, grammar is not a set of dead

____ has already been pointed out, grammar is not a set of dead rules.
A、As
B、It
C、That
D、Which
【正确答案】：A
【题目解析】：本题考查as引导的非限制性定语从句【要点透析】as引导的非限制性定语从句位置较灵活,可以位于主句前面、中间或后面,一般用逗号与主句隔开,但 which所引导的非限制性定语从句只能放在主句之后。其他两项均不合题意,故选A。