To her joy Della first earned the trust of her students and then of her colleagues.A. on

问题补充：

To her joy, Della first earned the trust of her students and then of her colleagues.

A. one

B. that

C. ones

D. those

答案：

A. one

(答案→)B. that

C. ones

D. those

解析：句意：让她高兴的是，Bella首先赢得了学生的信任，然后是同事的信任。这里需要一个代词指代the trust，that就可以指代前面的可数名词单数或不可数名词，one是泛指可数名词单数，ones是泛指可数名词复数，those是特指可数名词复数，所以选B。