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

脊髓发育成熟的重要标志是（）

A.重量的增加

B.长度的增加

C.髓鞘的形成

D.位置的上移

E.腹壁反射的出现