The poor little boy longed for a pretty toy train his father could never afford.A. thatB.

问题补充：

The poor little boy longed for a pretty toy train, his father could never afford.

A. that

B. the one

C. one

D. it .

答案：

A. that

B. the one

(答案→)C. one

D. it .

解析：句意这个可怜的小男孩渴望一个漂亮的玩机火车，一个他爸爸不可能买得起的玩具火车。这里需要一个代词做a toy train的同位语，one可以泛指前面的可数名词单数，that是特指前面的可数名词单数或不可数名词，the one是特指可数名词单数，it是特指前面的同类同物，所以选C。