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

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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。