The thought of going home to celebrate the spring festivai with his family was ___- kept him happy

The thought of going home to celebrate the spring festivai with his family was ___- kept him happy
The thought of going home to celebrate the spring festival with his family was ___- kept him happy while he was on his way back .
A that B all that C all what D which
was后为什么不能接which构成定语从句,为什么接all that