答案是：
that
定于从句~
先行词这里只能选that 不是表语从句吗？There once was a goat and a donkey是主系表结构呀！a goat and a donkey____lived on a farm.后面这半句~Ҳ����˵that���������ε���ǰ�������ǵ�~ a goat and a donkey