There were two small rooms in the house,( )severed as a kitchen.A the swhich B the smalmaller ofwhich 为什么选B

There were two small rooms in the house,( )severed as a kitchen.A the swhich B the smalmaller of
which 为什么选B

翻译过来就更容易理解了
房子里有两个小房间,其中一个是隔出来作厨房用的.
the smaller of which (was) severed as a kitchen
这是一个字语,修饰前面的two small rooms

是one of two rooms
所以要有of

限定性从句用of which