She was just about to explain she hadn't passed her maths test chiefly out of carelessness her uncle came.
A. her mother … when
B. to her mother why … that
C. her mother that … when
D. to her mother that … when
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A. her mother … when
B. to her mother why … that
C. her mother that … when
D. to her mother that … when
解析：explain/ announce等后面的结构是～ sth. to sb.或～ to sb. sth.，to 不可省略。