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

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(答案→)A. that
B. one
C. ones
D. those
解析：    
试题分析:句意：使她高兴的是，黛拉首先赢得了学生们的信任，然后又赢得了她同事的信任。that指代不可数名词；one指代单数可数名词；ones指代复数名词，是泛指概念；those指代复数名词，是特指概念。此句中被替代词trust是不可数名词，故答案选A.。