It is not until ______ that ______.

A．is a baby kangaroo four months old; he will begin to live outside its mother’s pouch

B．is a baby kangaroo four months old; will he begin to live outside its mother’s pouch

C．a baby kangaroo is four months old; he will begin to live outside its mother’s pouch

D．a baby kangaroo is four months old; will he begin to live outside its mother’s pouch