A lot 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.(2012·安徽，29)

A．as

B．it

C．which

D．this

对于普通员工的培训，有以下的阶段性变化规律，第一阶段是迅速学习阶段，第二阶段是缓慢学习阶段，第三阶段是（）。

A.心理界限阶段

B.再次学习阶段

C.学习停滞阶段

D.生理疲劳阶段