英语单项选择It was from John ,gave me a big key to __my dad.A.who;

英语单项选择
It was from John ,gave me a big key to __my dad.

A.who; understand B.that; understand C.who; understanding D.which ;to understand 这里是什么从句 还是强调句?
As best as Icould determine,I was standing in ___was once a terrible place,where they took you in and hurt you.这里填what