Ａlot of language learning,has been discovered, is happening in the first year of life, so

Ａlot of language learning, has been discovered, is happening in the first year of life, so parents should talk much to their children during that period.
A. as
B. it
C. which
D. this

(答案→)A. asB. itC. whichD. this解析：考查定语从句。很显然这是一个非限制性定语从句，故排除BD选项。as和which都可以指代前面整个主句。如果有“正如，象”的含义，并可以放在主句前，也可以放在后面，那么用as；而which引导的从句只能放主句后，并无“正如”的意思。句意：正如所发现的那样，大量的语言学习是在人的生命开始的第一年进行的。所以这段时期父母亲应该多和他们的孩子说说话。

就是这个解释